It's due to "Robert's Rules of Order" (Robert Storick's, that is). Some forum members have a history of just complaining about the rules, or starting arguments when a logical discussion is in order. There are 5 forums that are open to anybody, and many more that require (free) membership. All that entails is giving your real name and AMA number...don't know if there is a solution for non-USA participants...but a "Private Message" to Robert (aka Sparky) would be a good start.
As the instigator of the Judging Clinic Forum, I was more than a little surprised to find out that it's one that requires 'joining', but it does make some sense, too. I must add, that we're discussing Judging as it applies to AMA contests...and while we are coming closer to the FAI rules, they're definitely not the same. The pattern is the same, but the operating rules are a tad different. Reading the posts could benefit your flying and scores, I'd bet.
Maybe eventually, we will welcome and discuss the FAI Judging regs, but not at this time! I think it would be good to allow folks to "see" all the forums, but not post in all of them until they 'join'. I believe people who join under an alias are Robert's biggest worry and cause the most problems for us Terminators...er, I mean Moderators.

Steve