These white nationalists are Trump's people, his supporters, and he's emboldened them to make racism the "new normal". He doesn't want to lose their support, and the quote from David Duke regarding this entire incident says it all;
 
"This represents a turning point for the people of this country. We are determined to take our country back, we're going to fulfill the promises of Donald Trump, and that's what we believed in, that's why we voted for Donald Trump, because he said he's going to take our country back and that's what we gotta do," Duke said.

Nice job Trump voters.