Why not? They still have the right to speak for themselves or are you saying that the law should limit their own speech entirely removing their rights?
By your logic they would be free to post on other social media platforms but not host anything at all themselves. Think about that for a moment. Twitter as a company would be able to post whatever they want on Facebook, including posts negative to Facebook itself, without any restriction or repercussion but not post anything at all on their own platform. Facebook would be barred from removing any posts by Twitter or blocking them from using the service in any way.
While the Section 230 protection is intended to allow them to provide a service to others without being held liable for those that are using the service. In fact the law actually gives specific permission for them to remove or otherwise censor posts for almost any reason as long as it is done in good faith. Twitter T&Cs are clear about what they may do for unsubstantiated claims and glorification of violence. Section 230 is NOT about allowing free speech, rather it is about enabling companies to provide services without having to have an army of lawyers pre-approve ALL user activity to protect themselves legally. This would be unworkable and force many companies across the board from removing all publicly available forums to limit legal liability. Everything from 4chan to Slashdot to Youtube to Amazon review sections would be too risky to be allowed to exist. I don't see you arguing against Slashdot moderation systems but its actually harsher than anything Twitter ever does.
If anyone other than Trump had posted those things the posts would have been removed entirely and the account closed. Any apparent special treatment that Twitter has shown him is actually in is favour.
Anyone has at any time the right to just stop using any service. There are alternatives and people can quite happily live without the service at all.