Exclusive Use of Public Property

A number of flying sites in this country are located on property owned by a governmental entity such as a country or municipality. The Academy's liability insurance policy provides primary coverage for site owners. In addition AMA provides one million dollars of liability coverage for chartered clubs, sanctioned events, and individuals. AMA does not provide liability coverage for non-AMA members. Meantime it is not possible to obtain an AMA sanction and also sanction the same event with some other national modeling organization, similar to AMA, and offering insurance. The second sanction voids the AMA sanction. Why?