Note again the actual relevant text: "Congress shall make no law...abridging the freedom of speech, or of the press," which means Congress cannot make a law that permits Lieutenant Asshole of the Bumfuck Constabulary to throw your ass in prison because you ran your mouth (exceptions for threats, etc). All it means is you cannot face government persecution for speech, ie you cannot be disappeared like a Chinese dissident. Twitter, being not Congress, can ban you for any reason they like. Welcome to adulthood.