Couple of ways to think about the cops not breaking up the mob, legally and logistically.
Legally, there's nothing they could do to make the mob leave. The mob were invited guests of the person in charge of private property. Just like a cop can't come break up a hootenanny at your house because they decide you've got too many guests, the cops can't break up a hiphop after-party when the property owner wants them to be there. I've used the fire department to shut down stuff like this before, but you've gotta have all the people inside for that to work. This mob was outside. Until they break the law and until the manager wants them to leave, they can stay.
The next response is always, "Well you could go make it uncomfortable for them so they'll leave. Go park in the parking lot, turn on your lights, drive through the crowd until they all leave." Which brings us to the logistics of it.
What you've got are 400 angry, drunk, high black people wanting to cause trouble. As it stands, they're all in one place just causing trouble with each other. As long as they all stay at the McDonalds, they're only going to hurt each other (except in this obviously bizarre case of two white guys wading into the middle of them). Call it "Thug On Thug" violence if you will. They can act the fool and none of the regular, sane, tax paying, law abiding citizens are bothered by it.
But if you disperse that crowd you've got 400 drunk, high, angry black people driving around looking for a new place to congregate. And since nobody else will tolerate their crowd they're gonna split up. Instead of ONE problem area with 400 people, you've got FORTY problem areas all over town with ten people each.
Logistically, it's better for the town to let them all congregate and beat the hell out of each other than to turn them loose on the town.
