[mc32]

I'm guessing but I think it straddles a legal grey area, depending on what's carried out (how something's done).

In general one cannot expect any privacy while in public. That's been established many times over by the courts in the US. On the other hand, tracking recognizable people, I think without a probable cause, would be illegal. On the other hand if they just keep the footage and regressively backtrack activity after an activity was found to be illegal on the ground, then I don't see how using the generated footage would be illegal --and this would help law enforcement to some significant degree (i.e, someone was found stabbed on the sidewalk, no witnesses, let's backtrack the footage and track the attacker forward or backward).