Oops, you're back to legality again.

Let's drop the legal part. Here's a perfectly fair term of service: "We reject all content that accuses others of discrimination."

It's very easy for a white supremacist to pass this TOS and it's very hard for a BLM supporter. There's no incitement, no exploitation of children. Totally cool, because the internet isn't a necessary means of communication (according to you). Billionaires can totally afford (at least with leverage) to buy giant media companies. No worries. Right?

Well, no. The public good is not served by having important interests have a great deal of difficulty using standard means of communication. I don't know about you, but I get way more communication from the internet than, I don't know, the "town square" or a telephone or something. The internet is how we communicate these days. Maybe not for you, but for a vast number of people, the internet is it, including Facebook. The moral arguments for free speech apply in full.

Now, there are important caveats to the moral arguments (some of which I've written on elsewhere) specifically in the case of social-media; to a lesser (but nonzero) extent those apply to things like web hosting too. But at the level of detail we're talking about here, we need only consider the basic arguments.

(Note: I did not make a distinction between public and private businesses. Rather, I made a distinction between businesses who are selling access to means of common discourse, and those who are selling more specialized dispensable services. These days, practically speaking, flyers are dispensable, but an internet presence is not.)

(Additional note: Twitter and Facebook and so on have to an extent adopted content moderation policies because of government pressure. So it's also somewhat disingenuous to say that they're just businesses and get to set their rules. I think the Congress is correct to think that what happens there is in the public interest, but you can't then play the "business rights" card because you already supervened upon them.)

(Final note: the moral argument for affording rights to businesses is flimsy at best. People are important. Businesses are important only by virtue of the impact they have on people. So--don't try to play this angle too hard. I'll just end up objecting that businesses ought not have the same rights as humans. Note also--though it doesn't really bear on the correctness of the issue--the legal precedent seems to have been set duplicitously initially, though it has been legislatively reinforced since: https://www.theatlantic.com/business/archive/2018/03/corporations-people-adam-winkler/554852/)