They're closer in concept (and presumably in deployment scale) to mobile CCTV cameras than they are to police helicopters, and there certainly have been privacy issues with those.
I'm not so sure. With the latest electro-optical turrets a helicopter can be pretty far away and still get usable footage. During the Football World Cup I observed some police helicopters orbiting just about out of earshot during their operations.
I've also seen a demonstration of an experimental locally-produced surveillance camera (in this case intended for maritime observation) that was able to display the name on a small boat 20-30km out to sea. Granted, the camera was mounted on the top of a mountain to get the required height, but nonetheless the technology was phenomenal.
The difference between all the different varieties of surveillance technologies is starting to blur quite a bit.
There's a huge difference in price, though. The numbers will probably work out that you can throw up a hundred or so drones for the price of a single helicopter. With a single expensive platform, expensive optics and highly trained operators make sense.
