Twitter is free-as-in-beer, not free-as-in-speech. They can ban who they want, however they want. As much as people say “Twitter has become the new town square!” it hasn’t actually legally become the new town square; it’s still a privately-owned playground and they set the rules (however inconsistent they may be). If you created a site/app you’d give yourself a backdoor to ban who you want once they became a liability to you, same as Twitter.