[BizarroLand]

The difference is they could require proof of identity and American citizenship to be able to access the site, and the site itself could have rules that are enforceable to limit human generated spam without violating the first amendment. They would not be helpless to moderate the site just because of the first amendment, they would just need to pass moderation clauses for the site before going live with it.

In a similar vein with the fact that it's illegal to yell out "FIRE!" in a crowded theater unless there is actually a fire, they could make it illegal to advertise private businesses and make posters directly legally responsible for what they post.

If this does come about though, I hope they prohibit businesses and "Media Groups" or whatever from creating accounts or at least lock them into a walled garden that you have to voluntarily enter, segregating them from the general populace.