Going in backwards order:

1.) The website is the property of the owners, you are correct, but by extension, this forum, greatawakening, is effectively licensed by the mods, making the license of the forum their property.

2.) There's a difference between what action is being discussed on this forum versus what is happening in the mainstream/with the left/big tech/etc. I don't think I need to spell that out to you, but in case I do, here we go:

• The constitution grantees right to freedom of expression
• We also have guarantees of autonomy and property ownership/control.
• These rights go as far as they can without infringing upon the rights of another individual; we all should possess as equal rights as possible.
• When we sign up with these businesses engaged in censorship, or make a social media account, or anything like that, we are entering into a contract; an agreement. You might say that those contracts and agreements cannot supersede the Constitution; that is correct, and it goes both ways. While you may argue we are guaranteed freedom of expression, it exists only within the limitations of not infringing upon the rights of others. I cannot legally vandalize property of someone to express something to them. In that same sense, we are permitted "onto" their "property" (the platform) under the conditions of the contract. More on this in later points.
• Social media companies are enforcing their rules in defiance of their own contracts. They are, in those instances, violating the contract, and to some extent, the law (though in most of those instances, the damages are negligible).
• Big businesses are violating other laws about both censorship and monopolistic collusion; they don't just ban people in breach of their own applicable terms, they coordinately ban people for political expression. Perhaps most importantly, these bans and repercussions extend beyond the property of the platform. I could be banned on say, Twitter, for something I say here. I am not on Twitter's property when I say it; I am not infringing upon their rules, they are attempting to punish the execution and usage of my rights, outside of their "jurisdiction". Similarly, the punishments they deal extend beyond their "jurisdiction"; they seek to defame, shame, and mass-censor, through illegal collusion with their partners. Sometimes, they even seek to incite violence against those they ban. This overstepping of boundaries, coupled with their other infractions of law combine to make for an egregious assault on the constitutional rights. The so-called "slippery slope" referred to by many on this thread doesn't exist in that sense because there is already a well defined set of criteria for the violation of constitutional rights versus the simple utilization of property rights.
• To muddy this all, we have government intervention. When these platforms or companies become the government appointed (more like anointed) arbiters of their sector/industry, suddenly their rights over their own property change, because they have done dealings with the government that effectively make a percentage or portion of their company public property, by nature of receiving public funding and protection from competition. This is the biggest factor that defeats the use of the "go make your own" argument; in an instance where businesses were not colluding on grand scale, or where the government weren't significantly involved, the "go make your own" argument would be valid. It's validity is evident in the very existence of this (communities.win) website. Reddit did not possess complete monopolistic control, so whilst they were violating contracts and in collusion, creating competition for them was totally possible and doable, and it was done. As an example of difference, you can look at the Parler situation. By my understanding, Amazon terminated, possibly illegally, their contract with Parler, for reasons of illegal collusion, and with illegal government support. In that sense, the monopolistic conglomeration of big businesses is protecting its own, and downright preventing the creation of alternate social media platforms to an illegal level. Collusion from banks (who, by my understanding, have enormous government ties) sets an even greater level of concern, as it is the same problem but scaled up massively.

In a society with a justice system not totally compromised (like ours has been for a decent chunk of American history), it would be up to the judges and juries to determine the fine details as to where between conflicting rights the line should be drawn.

I want to comment on your mentions about practicality, as well: What is the difference between banning for usage of a slur versus banning for posting pornographic content? How about the difference between banning or censoring for reasons of utility (IE: off topic/unrelated to Q threads) versus slurs?

You say that "There's common sense censorship where something is inciting violence and that's bad"; I would actually advocate (once this is all over) for the legalization of speech that incites violence. Our country was founded upon speech inciting violence. The right to bear arms is pointless without the right to organize the bearing of arms. That said, just because I am for the constitutional and public legalization of that speech, that doesn't mean I'm for the right or ability to use that speech across the board; that is because I seek to obtain balance between rights, not just the over-exaggeration of one until it usurps and supersedes all others.