That's a fair point, they are under no legal obligation to provide a platform for free speech. That doesn't mean it isn't a valid thing to desire from your social media service of choice.
I haven't read Facebook or Twitters complete terms of service but I'm sure they allow for censorship. So as long as users agree to the TOS then it doesn't matter what words come out of the founders mouths. They simply leverage hypocrisy to their advantage with no shame.