The police may have guns, but they aren't the only ones.
Look how careful the two of them were until back-up came. It wouldn't have taken much for that to erupt into an ugly riot. I mean the only reason everyone hung around was to harass the cops, no?
When I worked in Manhattan and lived in Brooklyn (8-9 years ago) I used to dread getting on the train home when *school* just got out. The kids were in packs and always trying to impress each other with unruly behavior.