Because then they can use it as a justification for new laws to crack down on the "causes" of the behavior. It is a play taken right out of the playbook of their heroes, communists and fascists. Those two groups have done it in the past and the left tried to do it after Jan 6, covid and will try to do it again now.
IT is the same reason not one of our leftists has mentioned the Oregon football incident last Friday at a high school game. A player ran and shoved an opposing team player carrying an American flag during pregame.