Yes, if the government were to say, "People can express their views here", then indeed, you give them the power to say that "people can express their views here, too"! Oh, the horror!

The reason I bring up unions and OSHA is that companies do not, typically, negotiate with their employees or customers as equals. The company has far, far more power in almost every interaction--so much so that they can force people into highly disadvantageous situations because the alternative is unbearable.

The role of the government in creating a good society is, therefore, to figure out when to use its even greater power to prevent companies from abusing their positions of power (especially in the case of monopolies or near-monopolies). One way to do this is to explicitly enable another source of power (unions). Another is just to set limits on how abusive the company can be (OSHA).

So you rightly point out that the government should curtail its own potential power to limit speech because doing can lead to abuses. But you don't seem to want to notice (in this context) that companies can also have immense power over people, and that the same logic can apply.

(There's also the issue that the Congress summoned all the social media companies to complain to them that they weren't doing enough to restrict speech, so they have a long way to go before they're demanding that companies carry speech that they don't want to.)

All the examples that you bring up are straw men because in none of the examples are the companies providing the equivalent of a place for public discourse.

If the Catholic Digest were the only publication with broad reach, then maybe they should be required to carry a wider selection of material...but they aren't, so there is no justification for it.

Facebook, on the other hand, is exactly in the position of providing a digital equivalent to public discourse. So is Twitter. So is Medium, except it's too small to matter. In matters of free speech, therefore, you must run through the logic again with Facebook and again with Twitter and so on, and see how it turns out.

The principled "you can't make companies do things" argument is completely absurd because we already make companies do tons of things. Sure, we want to avoid overdoing it. We want to have a good argument for it. But it is not a priori the wrong thing to do.

One might run through the logic carefully and find that for whatever reason the advantages of free speech that justify the existence of the First Amendment are not applicable with sufficient gravity to Facebook speech, and therefore the rules should be different. That's fine, if it comes out that way. But you have got to have the discussion, have got to make the argument. You can't just skip to the conclusion that you feel is right, and shut down any discussion when others have different feelings or different reasoned conclusions.