No.

A monopoly has sole control and infulence over a given business domain.

1. The AMA is a club that one chooses to join to support model aviation.
2. If you want to fly RC you can do it without being a member of the AMA.
3. For those that look at AMA as a source of insurance, it is only secondary insurance anyway, and you can get coverage on your own (you may pay more, but you can get it)

Those are just the three that popped into my head immediatly, I'm sure more will follow since there is a lot of empty space to fill there.