Actually the law is pretty unclear about how far above your property your "air rights" extend. But honestly those cameras on these things are very wide angled lenses, so I can't see them being used in much of a spy sort of endeavor. At least not surreptitiously. They are LOUD, so it's not like you could sneak up on anyone. Think of hearing a thousand bees buzzing over your head and you'll get an idea of what they sound like. Even that little Hubsan I have is quite noisy and you can quite easily locate it if you lose sight just by tracking the sound.
Besides that, there is always going to be the issue of destruction of private property that will likely get the police involved in issues of this nature. Surely if someone shot my $1600 copter and camera out of the sky and I knew who it was, there is going to be some bad blood generated. The next quadcopter might be carrying some sort of revenge payload.....
