I hesitate because I don't want to give "people" ideas but the increasing sophistication of home built aircraft, complete with autopilot and GPS, must be giving airlines the heeby jeebies. They don't have to carry an explosive payload to be dangerous either.
I keep wondering why all these general aviation airports aren't closed and home built aircraft banned.
Sunfish, seriously if you ban the spotters you lose the passive community surveillance that stops the bad guys from launching a drone from that vantage point.