My guess is that the guys who draw up the plans may like to get paid for their efforts. 
Did the old magazines have people on staff to make the drawings?
Clearly the business was a lot different back in that era.
Warning, a long and perhaps rambling dissertation follows.Publishing a magazine was no doubt a lot more profitable back in the early days. The publisher could get good rates on advertising. The TV was either not in wide spread use, or not yet invented, and the only methods of mass advertising a product was Newspaper, Magazines, and Radio. Modelers flocked to the magazines that catered to the hobby, as it was the main media covering the happenings, the latest, greatest techniques, and designs of the day. Companies producing model related products also flocked to the model magazines because they were the best venue to directly reach their market.
Plans have been a part of model magazines almost from the beginnings of the hobby. Having the plans in the magazines was a big reason many bought that issue, and for some time, the plans included a scale, or some method to assist the modeler with the efforts to enlarge the plan at home.
Later they began offering full sized plans for those who weren't "skilled", or patient, enough to hand enlarge the magazine plans. In the beginning, the plans were relatively cheap, often costing the same, or slightly more than the magazines cover price.
Most plans of the day were on one sheet. Some were actually quite crude compared to today's offerings, but the modelers of the day were able to build some very decent models from them, filling in the details from their own practises, and experiences.
Today's plans are larger, more detailed, multi sheeted offerings, and in many cases far more accurate than in the past. Cad drawings are a whole new ballgame, offering extreme accuracy, and the possibility of laser cutting.
The construction articles were, and still are, one of the main backstays of the model publishing business. Having a good article about a really neat model meant huge sales of that issue.
Those numbers showed the advertisers, which magazine might best carry their message to the most potential customers. Publishers were always looking for new subjects and designs. Revenue from advertising helped defray, the cost of the publication to the modelers, keeping the cost of the products more in line with what the modeler was able to pay.
Some articles were solicited from builders of successful models, others were submitted. Today, that's still, with a few changes, how it happens. Whether an article was solicited or submitted, the publisher offered a payment based on the amount of work performed by the author.
The author could reap the highest rewards for the effort, by writing the article, doing the drawing, and taking all the pictures. In cases where the writing was weak, the publisher would use the editors to clean it up and make it more presentable. If the drawing was poorly done, or not up to standards, the publisher would contract a redraw to a draftsman who would redo the drawing to the standard the publication required. Editors liked, and still do, lots of good photos. If none existed, they could hire a photographer to take pictures for the article. Of course, all these options cost the publisher, and the payment to the designer was adjusted to reflect the costs.
It is no suprise that model magazines may have been a good money making business during those days, but as competition between the magazines, and other media increased, cost savings and enhanced income opportunities had to be found in order to keep the magazine afloat.
One augmented income opportunity was discovered in plan sales.
Plans were, and many still are, copyrighted. Large scale copy machines were unheard of, so the main method used to supply plans to the hobby was what we call "Blue Prints" using the Ozalid method many are familiar with from drafting classes in school. The other was to turn the process over to a professional printing company. The very best, but also more expensive method, is printing. One had to make a run large enough to get a cost discount on volume, so storage enters into the discusion.
Back in the day, plans were only available from the publisher for the reasons cited above. so the cost mostly reflected a modest profit to the publisher, after all, where else were you going to get the plans. Kinko's didn't exist.
Over the years, there have been a huge amount of changes in the business, and technology, that have affected the publishers bottom line. Those that didn't adapt, or could not, had to close up shop. Some very successful magazines have fallen by the wayside in the last decade or so. It's simply too much competition from too many areas for some.
Increased costs, and competition are the culprits.
Cost increases almost go without explanation. The cost of everything has increased, sometimes many times more what it was in the past. In the case of plans, consider the cost of acquiring, storing, printing, and servicing the plan sales.
Payments to authors have doubled in the last 10 years. Secure storage of plans is crucial. several decades of published design are no longer available from the publishers, due to warehouse fires. Ozalid or blue prints have lost favor with hobbyists, so today, most are printed, either offset, or, more likely, large scale copy machines. Have you priced Kinko's today? the average cost for one sheet is over $5.00.
If you've spent the bucks to purchase your own copier, and supplies, the cost isn't much less, especially when you factor in the price of the machine.
Servicing the request for plans usually goes through it's own department, staffed by at least one paid individual, whose salary must be paid out of the receipts from plan sales.
Now, let's look at what some of this competition is.
Magazine sales and subscriptions are in a downward trend. If we don't start supporting them, they will eventually have to close up shop. Competition comes from the huge changes in media fueled by the Internet. What the magazines used to do for us the hobbyists, is now done quicker, faster, and at less cost.
Forums, such as this one, disseminate information at literally the speed of light. I believe that many modelers today, seldom buy, or receive a magazine unless it's part of membership in an organisation. Their magazines are not wholly supported by advertising revenues, instead, they are subsidised by the organisation.
There are precious few independent magazines still being published.
Flying Models in the US, is one of the only magazines being published that cover all aspects of the hobby, and it's having problems due to lack of support.
Plan sales have become a large potential for what little profit is available to maintain the viability of the magazine, so it's no surprise, to me, that steps are now being taken to try and protect that source. Watermarking is one such method.
It does beg the question, "Why do this now? What use on the small published plan, does watermarking have on security?"
The answer is actually easy to surmise. Modern computer graphics programs are quite capable of enlarging the small published plan to usable proportions, thereby circumventing the possibility of a sale of that plan to an individual. Add to this the practise of individuals selling copied plans that were originally sold by the magazines. They are competing directly with the publisher, and at relatively no cost to themselves. If they use the small published plan as a base, there is absolutely no income received by the publisher. Even though most of these plans are copyrighted, the practise is growing. Costs of bringing suit on copyright infringement, usually cause such action to be deferred.
So, watermarking provides a method to discourage these practises. The published plans can show enough details so you can decide if you want to purchase the full sized plans. Very few modelers today have the time, skill, or will, to enlarge the small published plans these days, so encouraging the purchase of the plans helps the publishers bottom line, and believe me it's needed. The alternative would be to publish either only small parts of the plan, or none at all. Both of these options are counter productive, in my opinion.
In many cases these days, it's possible to purchase, on line, digital copies of published plans, including the articles, for a small cost. Though handy, and usable, this severely cuts into the magazines revenue.
Now, considering all the above, what might be some solutions to help the publishers? That presumes that the magazines are useful to you of course.
I believe PAMPA is leading the charge on possible solutions, by offering a special class of membership tailored to the internet. Magazine publishers may want to consider this avenue as well.
Converting to digital, and several offsite storage options make sense, and offer savings to the publisher, and eventually to the consumer.
Cost savings may be there for them to continue providing good service to us the hobbyists.
Either establishing, or perhaps better yet, aligning themselves with established forums, they can tap into an existing base of users. They may also be able to skim off articles and information, with permission, and a token payment, of course, that will help fill out the pages of their magazines, both electronic and hard copy.
Such movement, may also offer increased income opportunities from advertisers,, even if it's only providing links to advertisers web sites.
If a magazine is of value to you, consider subscribing and helping to build up their base, which makes them more valuable to advertisers.
We, the users of their products, should, as a first option, check their archives for the plans and services we desire. We should consider purchasing from the original sources whenever possible. Many plans and services are no longer available from the original sources, and those we can and should purchase from the providers of our choice.