Totally agree with what you said; especially with:

They are public fora. ...and therefore they are bound to respect free speech for the public.

Absolutely. Even if not legally, at least morally and ethically. (But we know they haven't much of either).

You make me think of the 2nd ROOT of the PROBLEM which I did not state in this post. Namely,

the absence of the RULE OF LAW

There are anti-trust laws in place - Sherman Antitrust Act (26 Stat. , 15 U.S.C.).

But the issue/problem is that it is not enforced like it use to be.

The lack of enforcement of this particular law, and many other laws in the USA, is systemic.
Whether you are looking at the blatantly illegal rigging of the markets by the Goldman Sachs, J.P. Morgans, et al., or war crimes, politicians and elites not being properly prosecuted, and so on, it is pervasive that this absence has destroyed the legal fabric that once held the nation together and upright.

Until there is restoration of the rule of law, nothing will change but will only get worse.

And when you have useless and spineless cowards like the Attorney General Jeff Sessions in place, it only proves that those institutions are greatly compromised as well.

I still don't get why people are not looking at these two underlying root causes of this fiasco.

Sad. Very sad state of affairs indeed.