Because an AMA chartered club would have all of the prerequisites that the AMA requires for insurance, i.e. a distinct flightline, separate from spectators, posted rules, and such. All of these regulations can be found on the AMA's website. I'm Not trying to imply that one cannot setup a spot somewhere on their own whereby they would be covered by AMA insurance, but do not be confused. One cannot simply go out, throw a plane in the air, and assume that he/she is covered just because they paid the yearly dues. It's not quite that simple.