This is something I try to mention whenever I get the opportunity. "Free Speech", as many people enjoy it, is only a protection from the government restricting one from expressing themselves no matter their opinion.
"Free Speech" is not a default state. Private organizations, as you said, have a duty to police the content posted on their platform. If one doesn't like the rules of a particular service, they can opt not to use said service. No one is entitled to be able to use Facebook, Twitter, or any other platform, just because they exist.