Earlier quoted context omitted.
The decision[0] gives different logic (quoting from page 85): If a firm’s core business is disseminating others’ speech, then that should weaken, not strengthen, the firm’s argument that it has a First Amendment right to censor that speech. In PruneYard, for example, the shopping mall was open to the public—but for the purpose of shopping, not sharing expression. So it was perhaps tenuous for the State to use the pub…
If you say "we are going to restrict ourselves [at the Government's insistence] to only moderating non-political speech" then someone will insist that some part of your non-political speech is actually political. And from some perspective they will be right. Then you get to negotiate with the government and the courts about what they consider to be political speech or not. And suddenly you no longer live in a country…
Yes, the government decides all the time, in a long standing and well studied legal history, about what is or is not speech.
In fact, I would go so far as to say that the primary purpose of the court system, with regards to free speech cases, is deciding what does or does not count as speech.