It's just a thing you have to do in order to effectively achieve non-partisan goals. An organization that picks a side on every political battle in the country won't be able to effectively focus on its core mission.
It's the same principle as being a non-religious organization. If the ACLU went around telling people that accepting Jesus as your lord and savior is the most important liberty, I suspect you'd agree that's very strange - even though many devout Christians do hold that position and think it's important.
My belief system makes white supremacy not some other, legitimate belief I oppose, but one that is beyond the pale because it threatens the core of my belief system (universal human rights and equality).
Since the (current) Republican Party is running on white supremacy ...