The 1st Amendment acts as a constraint on Government. Private companies are under no obligation to protect your right to free speech.
More to the point, your right to free speech isn’t a constraint on the private companies right to exercise theirs.
And even more to the point, @realDonaldTrump is (and has been described as such by its owner in court cases) a official communication tool of the Office of the President; it trumping the preferences of private media firm by whose resources it is relayed would be a direct, government violation of the right of free speech/press.