Stew, you have your finger on the pulse. The AMA insurance program was established decades ago primarily to permit site owners to have insurance, increasing the likelihood that they would be willing to allow a contest to occur at all. A flier can purchase liability coverage (homeowners or renters) which will pay claims made against him personally. Not so for the owner of the flying site, who also faces claims for allowing the activity on his property. In general, commercial lines insurance carriers have no loss data on model flying in any form, and are thus unwilling to write a policy for a site owner at all. There might be some situation where they would do so in order to protect a relationship with a major client, but the chance of running across it is minimal. The membership benefit of personal insurance is only incidental to the main purpose of the AMA insurance program, which is an inducement to site owners to allow flying in the first place.
About 12 or 15 years ago, there was an attempt to start a competing organization, the Sport Fliers of America. They promised insurance, and collected money for 'premiums', up until it came to light that no policies were ever written, and there were no reserves and no ability to actually pay claims. The whole thing collapsed. The moral of the story is that the AMA has a strong record of issuing legitimate policies and paying claims. I'd be very reluctant to try another insurance source.
One other thing. As matters stand, the AMA holds a 501c3 determination from the IRS that it is a legitimate charity, based on the scholarship program and the educational benefits of model flying. The insurance program which is a part of the whole enterprise is thus non-taxable. If the AMA wanted to divide the various areas of model flying into separate risk categories, doing so would involve risk that the IRS would find the insurance activity to be a for profit activity, and impose income tax on whatever 'net profit' might result from the insurance program as a whole. Let's call this 'audit risk'. I am aware that past AMA managements have declined to offer differing (risk adjusted) insurance rates because of this. Now it appears that present management has done so in order to try to increase market share in the parkflyer area. Audit risk should be considered whenever the suggestion comes up that this or that form of model flying should get a special deal on coverage. An IRS proceeding will be very complex, with numerous extraneous factors to be considered, but I would never suggest that the AMA should take any chances at all. My own thought on all of this has always been that all of us are in the same boat, and the last thing we need is internal warfare over what is really a very modest annual expenditure. IMO, what we all need in the first instance is the biggest baddest lobbying effort we can muster.