Trespassing. You have the right to assembly, but that doesn’t extend to anywhere, any time.
These protestors could protest on the sidewalk, or get a permit and do a planned protest in a public park, or even work with the city to close roads for a planned march. As long as they kept it peaceful, police would have very little justification to arrest anyone.
Instead, they are doing it on college campuses, or public roads without permission. And when they are told to leave, they refuse. At that point, you are trespassing, and the police are justified in arresting you.
Civil disobedience grabs far more attention than protesting legally. We’re here talking about their cause because it made headlines due to civil disobedience. But activism has its costs.