When the police are impotent in the face of anarchists and students causing criminal damage then it sets a bad example.
You find a lot of these left wing students will say they are standing up against capitalism etc, and focus on 'police brutality' if the police try to get in the way and so on, and enjoy getting one over on the police by showing what they can do.
I wonder how they would view it if they turned on the TV and saw a load of skinheads smashing up banks and businesses because they are associated with Jews, or immigrants. Would they be so keen to slag off the police for taking steps to intervene?