Facebook started as a way to connect with classmates. Instagram started as picture sharing. Twitter started as a way to message friends.
At what point were those companies suddenly "in the business" of facilitating discourse? Who decides when they are? Why should we have laws that force a company to behave a certain way just because it's popular?
Thankfully, the US has clear laws about this, and private corporations can permit or reject users as they please, as long as they don't do it solely on the basis of gender, race, or religion.
Further, I can't believe that the discourse that is 'stifled' by hate speech terms and conditions on platforms like Twitter was ever valuable in the first place. I can't believe it's more valuable than the discourse of those who are silenced by harassment.
I can't say that they should disallow illegal speech because governments get this wrong, too. Like how many countries have laws forbidding hate speech. Those laws are as abhorrent as the speech itself because it's stifling discourse.