It's not often Trump is right, but he was right this time.

Left wing activists think there is a duty to confront; they also think the right-wing activists have not such right.  It wasn't enough to simply allow those right-wing nutbars to have their legally-approved demonstration and then critique them the following day.  They sought to remove their voice.  

Not all counter-protesters were there to pick a fight, but a significant number were.  Hate is hate, regardless of which side one is on.