I don't think you understand free speech in the US context, Twitter is protected by the first amendment, it itself has freedom of speech and of association. It does not have to allow anything it doesn't want on its platform. It could ban everyone wearing blue in their profile photo, it is THEIR platform. If Twitter COULDN'T ban anyone they wanted under US law THEN it would be a violation of free speech.

And that wouldn’t be an issue if twitter wasn’t so dominant for this form of communication between positions and people. It’s because it is so entrenched without a meaningful equivalent service that this becomes significant.

Could you explain how it is dominant? It is literally one tenth the size of Facebook. It is not even one of the top 10 social networks, currently at #17 https://en.wikipedia.org/wiki/List_of_social_platforms_with_...

It’s hard to draw clear lines between domains, but I think for public representation of official organisations, there isn’t any other. So for this purpose it’s dominant.