On American soil, law enforcement is required to always try to take suspects alive. Only if the suspect posses an immediate and potentially fatal threat is lethal force justified. Drone strikes are unacceptable because they currently only have lethal weaponry, which means that any situation where they would be practically useful couldn't be justified legally. Reconnaissance is the only reasonable use of drones in their current state. A taser or judo-bot for subduing suspects non-lethally would be okay if someone invents them in the future.