US progressives as a group are richer, whiter, and more educated than the average American, and so penalising posters (even just socially) for ideological non-conformity contributes to an overly-white message board. Lots of posters have left or been banned at least partly due to ideological disagreement, and I think POC are more likely to leave for this reason because fewer hold the one approved viewpoint™.
Kind of ironic since progressives are strongly opposed to racism.