If the cops issue a lawful order to disperse, and you refuse it and subsequently get gassed or otherwise hurt, do you have any grounds for a lawsuit (juicy video notwithstanding)? Absent unreasonable use of force or some other element? You don't often hear of claims getting successfully pursued in cases like this, although I suppose it's possible.
These things always seem like a no-win situation for the cops. Act too gently, and you're accused of standing by while one group attacked another or property got destroyed. Act too vigorously, and it's called police brutality.