> A comment section on a blog presents itself as a public space. Does this mean nobody can moderate comments anymore?

A blog comment section usually presents itself as a curated space, and the flip side is that blog owners take some level of responsibility for the contents of their comments.

You can be a private club, or you can be a public space. But you can't be both. We make that kind of distinction in the real world too.

> Or if the rule only applies to political expression, can spammers just adopt the formula: “I love candidate $X because he buys viagra at http:yyy”

Of course not, come off it, judges and juries are not actually complete idiots.

> Or, if the rule is that big enough sites suddenly become “town squares”, that seems like a major disincentive to American companies to grow. Others would walk in without these limitations, like Chinese already did with TikTok.

Then ban those sites from the US, like India does. If you can't support US-style freedom of speech then you shouldn't get access to the US market.

A direct analogue is the concept of an easement. If you get large enough and enough people use your site, that is when the public square doctrine would kick in. Similar to how eventually if you let people keep walking through your yard it becomes public use land.

Usually you have multiple chances to fight it. The owner of a square continues to own it because it makes them money.