Public discourse was and should always be in the private domain. Anything else is totalitarianism by definition.
Public discourse squarely belongs to the public, and hosting it must be perceived and intended to be a kind of public service; otherwise we can just go back to the good old days of gathering and debating in public squares.
You can't assume the government wouldn't censor based on the exact same criteria.
An entity with control will exercise it.
When it's a private entity there's a theoretical other place you can go. When it's public (government) there is fundamentally no other place you can go.
"Public" and "Private" and "Government" mean different things at different times.
In this case I think you mean Public Discourse is not controlled by the Government. And that's where a lot of confusion is coming in.