The argument is "race as used in the U.S. is a social construct that is incorrectly correlated with some physical traits. So lets avoid bringing up someone's race when trying to explain their behavior and/or intentions."
Just because someone has dark skin doesn't automatically mean they are "black" because black is more of a cultural thing than a race. It's incorrectly attributed as a race in the U.S., and in many cases it does coincide, so someone that looks black is probably black, but not everyone. This is more so the case with white people. The white label is a huge generalization on many sub-cultures and races, not just one. So it is inefficient to try and explain behavior with these labels that are incorrectly seen as race.
To further expand, yes there is a problem with racist people, but what's the point of saying they are "white" racist people? They're just racist. No need to be further promote the same mis-categorization that racist people love to use. Racist people are ignorant and/or crazy, and we can continue analyzing them and fighting without mentioning anyone's race.