They've already given him tons of free press for a week now by freaking out. Instead of ignoring him, treating it like the fringe ideology it is, they want to treat him like some powerful leader. This only gives Spencer credibility. 

The only reason anyone is even talking about Richard Spencer is because the left is obsessed with the idea they are fighting Nazis. The side-effect of it is endless free marketing for what would otherwise be a fringe ideology. Literally no one on the right is giving Spencer a platform to talk.