You like to paint with a wide brush, don't you?
Lived many years there myself. I find that usually a general crowd does not act in concert, when in hind sight they might have. There are many cases in which the armchair know-all press cite crowd inertia. Often there is only the one or even two persons who will intervene.
I was a security guard in a large mall in Canada. A two bit low life punk tried to drop kick me to oblivion. 300 watched. Only two men tried to help. Not to cry the blues, but I will wager there are some American security personel out there, had a similiar experience.
For laughs- sitting in the open fast food area,burly middle-aged men afterward, advised me just how THEY would have flattened the punk.
Perhaps there are certain cultures and different American States, where the crowd pitches in- have not heard of them though.