I for one hope they do not make flash mobs illegal. I love seeing the "improv everywhere" type stuff. I would hate to see a group of people singing a hearty song in a park get dispersed by flash bang grenades because it's a "flash mob."
Don't lump every flash mob into the category of destructive, malicious, terrorist activity, whatever else. Some people really do use them to cheer up other people's daily humdrum lives.
But more on topic, what happened in the OP is terrible and I wish that no one get caught up in something like that. I would call something like that a riot.