Advancing progressive politics necessarily requires upsetting the status-quo, whereas if someone is a social-regressive then simply adhering to the status-quo suits their agenda just fine.
...so then some places want to avoid that image so they do allow discussion on controversial topics with the constraint that people debate things civilly - so far so good - except those same places want to void an image of partisanship - and in the spirit of freedom-of-speech they'll allow discussion on any topic (again, provided it's done civilly).
...which leads to that place or community falling victim of the paradox of intolerance (as, by their own rules, those communities must allow for the advocacy of genocide and unspeakable crimes provided they're advanced by an individual who conducts themselves with politeness, while their debate opponent might be a gay, black disabled Ethiopian Jew who is rightfully concerned for their own life and future who may utter a swear-word a bit too loudly and suffer censure for doing so).
As far as I can tell, the most workable solution to that is for the bounds of the Overton Window to be explicitly declared by the bossses/mods/admins - and in doing-so instantly open themselves up to accusations of partisanship, especially if extremists take advantage of people acting in good-faith.
I believe in most places the current Overton Window permits discussion and advancement of communist utopian ideals but not far-right ethnonationalism - assuming those two are somehow equivalent - and if Facebook - or any other place - had a similar declared Overton Window policy then it can be said they're biased towards the left, which is great fodder for the pundits on America's most popular right-wing TV news channel.