But proponents of drones being used in the United States argue that they're a lot like helicopters, which have been in use for decades.
In a blogpost, the American Civil Liberties Union argues that there are important distinctions between the two types of surveillance tools.
For one thing, helicopters are far more expensive than drones. "Manned helicopters and fixed-wing aircraft are expensive to acquire, staff, and maintain. A police helicopter costs from $500,000 to $3 million to acquire, and $200-$400 an hour to fly," according to the ACLU.
That means mass use of helicopters by police departments is prohibitively pricey.
By comparison, the ACLU argues, "it’s easy to foresee a day when even a professional police drone could be acquired for less than a hundred dollars, including maintenance costs."
Drones can also get places that helicopters can't, meaning they run the risk of becoming more invasive, producing greater privacy concerns, the group said.
"Even the smallest manned helicopter can’t fly into a garage or hover unseen outside a third-story bedroom window," the post said.
Drones can also hover silently for days, unlike noisy helicopters that can't stay in the air for nearly as long.
Then Salon columnist Glenn Greenwald has voiced similar concerns in the past:
The fact is that drones vest vast new powers that police helicopters and existing weapons do not vest: and that’s true not just for weaponization but for surveillance. Drones enable a Surveillance State unlike anything we’ve seen. Because small drones are so much cheaper than police helicopters, many more of them can be deployed at once, ensuring far greater surveillance over a much larger area. Their small size and stealth capability means they can hover without any detection, and they can remain in the air for far longer than police helicopters.
Greenwald also notes that, while weaponized drones have not appeared in the domestic sphere, the surveillance drones that have can be easily armed.
Domestic drones are just one example of police militarization, a process by which domestic law enforcement agencies become more like the armed forces that fight abroad.
The Huffington Post's Radley Balko reports that this phenomenon has been going on "for about a generation now."
Right now though, drones are perhaps the most visible and controversial aspect of this longtime trend.