> What I find more interesting is that when that same argument is used in other situations, such a small businesses deciding to open during the lockdown, it is routinely down voted and shouted down.
The key concept is that it would be a violation of Facebook's First Amendment rights to forbid them from banning their users (i.e. force them to carry their messages). It would be compelled speech/compelled association.
Your example about businesses defying covid lockdowns is a completely different thing. Being a private company is not a license to do whatever you want: Facebook still has to comply with the law, as do those businesses.
If you're looking for something that looks like a contradiction, the one you're looking for is antidescrimination law: why can Facebook ban users of type X because it's a private company, but it's illegal for a wedding cake maker to refuse customers of type Y? If you're looking at it simply as a legalistic First Amendment thing, the situations look they should be the same. If they're not, it's because it's not a simple First Amendment thing, and other principles and context have to be balanced with First Amendment principles.