Actually, they're able to. I saw an NPC who punched this cop, the cop got up and aimed his gun at it, he didn't shoot at first but then the NPC started running thus promping him to shoot because the NPC was ''resisting arrest''.
Some time later I crashed into a Phoenix near the Mission Row PD almost killing a couple of cops, this made them draw their weapons at the car and one of them got near the door yelling at the driving to get out of the vehicle, the thing is.. the driver was dead.
It's really how the Police AI reacts to crime. If I punch somebody while being witnessed by cops then i'll get two stars, same applies to the NPCs.
I don't know what is it with the ''Hi'' bandwagon.. anyways, I got a wanted level for seriously I dunno. Knocking out when ''nobody was watching'' maybe? Seriously in all honesty I haven't gotten a crime for any ridiculous thing.