Arresting a person in a pissed off crowd isn’t exactly easy.
What we see in the video is the end of police enforcing dispersal of the crowd. We don’t see the repeated non violent orders to disperse that preceded this.
The people in the video participated in an illegal protest and ignored repeated police orders to disperse.
As far as police violence goes, the video isn’t terrible. You even see the cop help up the guy on the ground.
Press secretary would be perfect.