I think you’re suggesting that the free speech argument is overblown when it comes to online discussion forums, and I agree with that - they can, should and do set house rules.
The problem that Facebook faces is that it has become bigger than government, and so people expect it to obey similar rules regarding the protection of free speech, but Facebook does not have the power to make actual laws, so it is not required to protect free speech on its platform. Free speech is intended to protect democracy from governments that would enact laws to limit dissent. It is not relevant to Facebook.
The argument that a product whose business model is based on artificially weighting communications between disparate groups for profit and engagement should protect peoples right to inject whichever provocative messages they want into it in the name of free speech seems like a perversion of the principal to me, because the result is an erosion of democracy and human rights as we have seen.
They absolutely should ban people for posting stuff that doesn’t align with their brand, and in a free market those people can find another forum for discussion with more permissive rules. Preferably an open, decentralised network that does not rely on ad revenue or have any incentive to automatically promote the growth of groups of algorithmically selected like-minded individuals.
And yes, I am ignoring all the good things about FB here. I don’t believe Facebook has amounted to a net good for society, however good its intentions.