Earlier quoted context omitted.
You're missing the part where no one seriously wanted to stop it. As you point out these kind of excess happen all the time at festive events, someone drunkyard will kick a car, steal a street plate or deface a poster. But it stops there, as a random mischief. People piling up on the same thing isn't random or just a natural behavior, and nobody telling them to just move on goes along that line.
People burned up a Muni bus after the World Series, and no one stopped them. And it was certainly a collective thing, not a lone miscreant. Vehicles that are owned by large apersonal organizations seem like a common target in this type of thing.
France had that in spades in some suburbs, where bus is the most viable public transportation, yet bus drivers would get harrassed or straight beaten up.
I think it comes down to the only "authority" figure they deal with on a daily basis, and it's so unfair to bus drivers who are usually the most duty driven and community conscious people on the block.