What if a large platform has strict posting rights to cultivate a specific community standard that has nothing to do with the law? If my power-washing forum gets to over 10m users I'm now legally obligated to publish everything that doesn't violate US law, regardless of my site's content moderation policy?

What about large platforms based in other countries with a strong user base in the US, or vice versa? Do you force tech companies based internationally to comply with US law for content moderation purposes globally, or just for US residents? If not, what's to stop Facebook from moving their offices to Canada and continuing their business model as-is?

I fail to see how this is an even remotely reasonable assertion.

Sure, it's fine to have that belief. Just, to make it work in a world where the Citizens United precedent exists, you really need a constitutional amendment. Or a supreme court willing to make a really creative interpretation of the constitution and precedent landscape. This isn't a normative statement about what ought and ought not to be. It's just an observation of what is.