Even the "ideal" of free speech refers only to speech on public property outside of the control an organization. There is no ideal of free speech that says a) you should be able to say whatever you want on private property, or b) that organizations should sanction members for speech they do not approve of.
Twitter is well within the ideal of free speech should they choose to kick Donald Trump off their site, on the grounds of a) not liking what he uses their private property to say, and b) wanting to sanction him as a member (i.e. user) of their organization for what he says.
Of course, the ideal of free speech and what's written in the constitution are synonymous, as far as the American ideal of free speech is concerned. Different nations have different ideals.