Social media companies don’t lose their right to exclude by having allowed you to sign up and use their services. They can still exclude you (read: ban you) pretty much at-will subject to their terms of service (so pretty much always at-will) and it’s not disingenuous to point out that yes, this is in fact their right as the owners and operators of the service.
Other service providers the facts may be different. If you have a contract with a service provider, you have a lot more in your favor under a negotiated agreement than the people that clicked “I Agree” the last time a social media company changed their TOS.
You're talking about what is currently, I'm talking about the philosophical foundations of what could be. Analogous reasoning could be applied to SaaS (Software Augmented with Arbitrary Surveillance). Even if we were to consider individual private property sacrosanct, this doesn't need to extend to corporate property employed providing public-facing services.
For the same reason that landlords have to continue housing people doing things that the landlord disagrees with. Because they took what was their private dwelling, offered it to the public as housing, and invited in a new party that has their own rights.
I don't really have a dog in the "deplatforming" fight, because I think the sooner we move to decentralized protocols the better. It's just utterly disingenuous to pretend that corporate property deliberately employed to perform services for customers is exactly the same as individually owned private property.