Because it presently forces anyone that wants to fly to go to a FRIA site and that requires an AMA membership, something that many won't go for. You know my opinion of the AMA headquarters, it's a cash cow that needs to go on a serious diet, to put it mildly. Many of the programs are out of date or unneeded or, as we've discussed, ineffective, but I digress. Until the AMA learns to spend the money they get from the members in a manner that isn't wasteful or self serving(i.e. on pet projects), many won't consider joining and will fly where and when they want, regardless of what the FAA says.
I would have no problem going to a site where I would be asked to help maintain it to be able to fly. I wouldn't even mind a fee for entry if that was used to cover things like the previously mentioned maintenance or utilities. For example, I spent this past weekend as a paying participant at a festival in Albany. Even though I wasn't part of the staff/committee, I spent part of the weekend pushing one of the featured entertainers around in a wheelchair. There was someone assigned that job but I ended up doing it anyway as the person assigned had a medical issue and wasn't able to do so without pain and difficulty. Needless to say, it needed to be done so I did it.