“Yes, race is real, race matters and race is the foundation of identity.”
To some extent this is true, but there's an impression (a correct one, in my opinion) that people speaking about white nationalism want America to be predominantly white, as if whites own America. They don't see other racial types as equals, nor do they desire them to be a majority or hold positions of power.
It's this anti-humanity position that earns them this sort of reaction. People feel they need to demonstrate against this type of thinking. Sadly, demonstrations of any type are prone to turn violent by those who feel emboldened by the crowd.