The First Amendment to the United States Constitution reads:
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
This is freedom of speech: you can say what you want, and the government won’t sanction you for doing it. So if you want to go on national television and pimp out your new historically revisionist movie, and there call homosexuality “unnatural,” “detrimental,” “ultimately destructive to so many of the foundations of civilization,” the government won’t stop you and won’t sanction you for it after the fact. But you don’t have a right to be free from criticism from other people who disagree with you. It’s sort of like when you say something amazingly offensive and stupid, and lots of your advertisers decide they’d rather be associated with someone other than you.* That’s also not a First Amendment issue. (You can argue, I suppose, that tu quoque is an appropriate response instead, but don’t be surprised when it doesn’t work; and by the way, keep staying classy, Michael Moore.)
Although this scenario is playing out on the right side of the spectrum now, there’s players on the left who will drop the “Private criticism of me is suppression of my free speech rights” card on occassion too.† And it’s just incorrect. Freedom of speech does not mean freedom from criticism. Freedom of speech means freedom to criticize. If your view is silenced, perhaps it’s because everyone else in the world can tell that you were wrong, that you didn’t know what you were talking about, or at least that you were out of line. If they subsequently ignore you, contradict you, lampoon you, or otherwise heap abuse upon you, that’s the risk you took when you entered in to the arena of public opinion.
While the objective of the Constitution’s guarantee of free speech is not necessarily discernment of the truth, it is a very fine side effect of it indeed. If we learn, through allowing people to speak their minds freely, that in fact they are fools, bigots, or worse,‡ well, we’re all actually better off for knowing it.
* Nearly three dozen of them, according to one not-exactly-unbiased source.
† E.g., When Roseanne Barr decided to mangle the national anthem and mock baseball players at a baseball game, IIRC, she played this card; this was also the point that her entertainment career jumped the shark.
‡ What personal shortcoming, reasonably exposed during public discourse, could be worse than bigotry? How about sociopathy?