Simpler, but I think incorrect. Liberals tend to take positions which include going out of their way to fairly treat people that they very well know don't agree with them. As exhibit A I would take about half the stuff the ACLU does, and as exhibit B most of the efforts to support multiculturalism.

That is not to say that there are not liberal tribes - there clearly are - but the mores that they have push them away from straightforward acceptance of tribalism.