They don't "have more freedom in this area": they're not bound by it at all. They can ban whatever they want from their own platforms.

The first amendment, like less encompassing provisions in other liberal countries, was arguably intended to forbid prior censorship, whereby opinions had to be vetted by some authority before publication. It was expanded in its interpretation to other kinds of censorship, through the idea that a chilling effect on speech was comparable to a priori censorship.

I wonder when someone is going to figure out that obscenity laws violate separation of church and state and are thus unconstitutional.

Is allowed under some interpretation of the first amendment. It is not allowed by the first amendement per se.