> you just described a private investigator.

In most states that requires a license with actual professional standards being met to obtain and maintain one. It does not entitle you to harass someone.

> stalking requires some kind of menacing or whatnot.

Repetition, threats, and fear. The standard is "would most reasonable people perceive these actions in the same way?"

The better question is, in the cities that have installed flock, is the crime rate actually down? And can we make FOIA requests to see how often and for what the police have queried the system to receive data? I may not be able to challenge the existence of the system with a TRO but I can constrain police use of it; hopefully, to the point it is no longer economically viable for them to operate it.

The license is for selling a commercial service to the general public; the underlying activity (following people in public places) is lawful.

It's how much of journalism works: they're labelled "paparazzi" when it's negative-sentiment, or conversely "investigative journalists" when it's positive-sentiment. If you outlaw private citizens observing happenings in public spaces, you outlaw much of journalism. The targets of journalism, practically by definition, do not consent to being observed, analyzed, and reported on ("Journalism is printing something that someone does not want printed. Everything else is public relations"–Orwell).

And certainly the first to be arrested—ironically, if it was entities like Flock you meant to target—would be journalists observing, in public, police and LEO actions. There are a lot of powerful people eager to outlaw that today.