Courts might rule that a trespass by drone is unlike a trespass by a pet. Drones can carry cameras, so a drone flying into a backyard isn't just trespassing, it's a threat to privacy. It's also possible to put a weapon on a small drone and attack someone, even if it hasn't happened yet. If a person shot down a drone because they believed themselves to be in danger (or actually were in danger; state laws differ), and not just because it was annoying them, then it's possible that self-defense is a valid justification.