It also depends on the crowd one's dealing with. In the neighborhood I grew up in, I wouldn't hesitate for a second to confront the partygoers, and would feel completely safe in doing so. That neighborhood in Miami is another story altogether.
And I agree with everything you said, however, there are a lot of people out there that belive the are required to give their name to file or make a complaint. And again, most people that get the cops called on them, generally have a good idea who the ones doing it are.
This guy just went sideways, possibly after he was threatend or confronted at the door by some macho drunk. He may as well shot himself too.