Or does the fact that it's in public (or at least the ones that are video taped in public) make it legal?
It just seems more creepy here because it is 'in your face' but actually, if you think about it, it is more creepy to be followed and recorded by a dark room full of hidden creeps when you don't even know you are being recorded or who they are.
There's also systems like ShotSpotter - http://www.shotspotter.com/ - which are essentially recording, in high fidelity from numerous points within a city, the sound of everything that occurs.
Acccording to this NYT piece the system can pick up“doors slamming, birds chirping, cars on the highway, horns honking” as well as conversations.
http://www.nytimes.com/2012/05/29/us/shots-heard-pinpointed-...
Depends on if it is a one or two party state, and whether or not he constitutes a party. In Oklahoma, for example, if you and I are on the phone I can record the conversation without notifying you. In some states (New Hampshire, I think, off the top of my head) all parties to the conversation must be informed.
If he is partaking in the conversation or within earshot, he might be considered a party, and might be okay.
I am not a lawyer.