The problem is that the protest is illegal, but totally lawful.
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the government for a redress of grievances.
Peaceful protests are totally lawful. Just wait until its not peaceful anymore.
Illegal, but lawful? Hmm.... Actually, no.
To stage a large protest typically requires permission from the locality you want to protest in. If you don't have permission to use public areas for your protest, the police can definitely drive you out. That's exactly what happened in Pittsburgh today. The protesters didn't have permission to be where they were, the police asked them to leave, and the protesters refused. Then (allegedly) they started throwing things at police, which prompted police to use tear gas, etc.
What gets me in the videos I've seen, is most of the people look like average college kids and aren't holding signs or protesting. They're just gawking, but they still refused to leave when directed by police. Then they get all huffy and puffy when the cops gas them, when they could have avoided it by simply leaving the area. People don't seem to realize that once the cops put on their riot gear, they aren't playing games anymore. They are in it to win it, and if that means some rubbernecking college kids get tear gassed, well, them's the breaks.
Like I said before, I'm not a fan of law enforcement - but when a cop in riot gear tells me to clear the area, I move my ass, pronto. Is it just me or do college kids have some cockeyed sense of entitlement?