Twitter is acting entirely properly, the problem is in relying on twitter
As many people have pointed out, it's twitters platform and if they don't want you on it because you don't play by their rules then they are entirely within their rights to kick you off.
The problem here is that Twitter, a private company, has been handed effective control of a global infrastructure that should raise so many red flags that no company or organisation should rely on them for anything more than free advertising.
Instead we need an infrastructure like web, or email, where anyone can put up a server and anyone else can choose to either listen or not listen. The government would then have its own server under its own control where the president can post whatever he likes, and it's up to you as an individual, or your chosen trusted service provider, whether they federate (listen to) the government server or not.
I *think* Mastodon is in this model...