You've already gotten a lot of replies about difference in degree, but there's also two differences in kind: one with visibility, and one with capability.

A police cruiser sitting at an intersection is visible. People notice it and its presence changes their behavior. They may choose to avoid certain routes through a city where surveillance cruisers hang out. Flock, and similar surveillance systems, are far less noticeable. Many, many people had no idea they existed a year ago when there were already 80,000 devices installed. Flock has been hot in the news cycle lately in part because tons of people are still just beginning to learn they exist. Whereas a police cruiser on a corner is overt surveillance, Flock is a "creepier" incarnation.

There's also capability. In principle, an officer parked on a corner is capable of responding to a variety emergencies. Some people might feel that the presence of the officer there was a net good for public safety. Heart attack in the neighborhood? Your local surveillance officer can start CPR. Older person with late-stage dementia has just walked away from home? Your local surveillance officer could clear that neighborhood.

But these surveillance systems have only one capability: surveillance. They can't render aid. They can't even prevent a crime; they can, at best, discourage it, or maybe aid in the apprehension of someone who committed a crime -- and didn't take adequate measures to avoid detection.