In most mass protests there isn’t a source of authority that could decide who gets to be in the place the protest is. There’s organizers and speakers who can ask them to leave, but if the problematic attendees don’t just go along with that they don’t have any inherent authority to kick them out. That doesn’t mean saboteurs don’t get kicked out, but it’s more in the form of a bunch of people self-organizing to shout them down or put a human wall between them and the rest of the group. The student protesters have done this a few times when opponents try to infiltrate their ranks.

But it’s not something you pull together because someone’s sign isn’t well tuned or one of hundreds or thousands of voices is demanding Exxon Mobile be immediately shut down when the actual goal is ending fossil fuel subsidies. Partly because you don’t want to fracture a movement by kicking out your allies and partly because this is an entirely social endeavor organized among strangers and it’s hard to get that arranged if people aren’t instantly convinced it’s necessary.