Or you could say the cop doesn't want an umbrella in his face blocking his view.
The umbrellas are used against pepper spray. They aren't there for violence, they are to protect rioters from pepper-spray.
Fuck them, fuck their umbrella.
I get why they have the umbrellas. However, when you are in the front line right next to the cops and pepper spray isn't being used at the moment don't stick the umbrella in dudes face blocking his view of what could be going on right in front of him.