Because in many states (maybe all?) they can be.
Remember, websites are only given immunity if they fail to remove “sueable” content.
Will they (hacker news):
a. Prescreen all comments, like a publisher prescreens manuscripts, or
b. Just do away with user-submitted content, or
c. Let everyone say whatever they want in an orgy of freedom with no moderation whatsoever?
Privately owned media sites will never, ever be a public square just because people use them the same way my yard isn’t a public square because you can walk across it on the way to ring my doorbell so you can ask to put a campaign sign in my grass.
Is the President of the United States of America sharing a video calling for the death of his political opponents a “sue worthy” event? What if he gave a speech saying that? Or published a presidential decree calling for it?
Some people see bigots saying “whatever dude it was just a satire joke” for what it is: bullshit.