Congress shall make no law blah blah blah, abridging the freedom of speech.
That's what the Constitution says. I am pretty sure (hyperbole) that the Constitution doesn't say that each American has the right to say whatever he or she wishes. It merely says the government may not limit what people say.
Where precisely does it say people have the freedom to say whatever they wish to say?
If the government may not abridge the right to free speech, who else can?