Free Speech and the Defamation of Religion
The general rule in the U.S. is that you’re allowed to say what you want to say. The corollary is that you can respond to whatever somebody else says however you want, so long as you don’t break other laws. You can debunk, boycott, or ignore what somebody says. But you can’t punch him in the nose. Even if you want to.
So when we’re talking about what can be done about travesties like this film that is inciting protests around the world,, the answer is 1) it’s pretty clearly protected by the First Amendment, and 2) that protection is deeply embedded in the Constitution and we’re not going to change that.
There are some exceptions to the general “say what you want” rule—but none of them apply.
Defamation is an exception—but in this case it means something reasonably specific. Essentially, if you go around saying or writing false things about a person or group that harm that person or group, you can be sued, provided that person or group can bring a lawsuit against you. In this case, 1) historical figures are going to have trouble doing that, and 2) that person or group may be entitled to any money you have—but they can’t stop you from saying something before you say it. So even if the folks behind this film said something specific enough to support a defamation suit, the can only be sued for whatever money they have. They can’t be locked up.
Speech that incites violence is another exception—but in this case you need to be encouraging people to go out and do something violent. “Go punch that guy in the nose” is an incitation to violence. Being such a jerk that people come and punch you in the nose is not.
There are two other potentially applicable exceptions. One proposed exception is an exception for hate speech. This one is inapplicable because it’s just a proposed exception in the U.S. pretty much nobody recognizes it, even if some people think it should be an exception.
The other exception is an exception for “fighting words.” The “fighting words” doctrine says that it is legal to prohibit a particularly obnoxious Jehovah’s Witness from calling a town marshal a “god-damned racketeer,” provided it is 1932. The general idea is that if you’re saying something so offensive that it’s likely to lead to immediate violence against yourself, there’s a way for a town to ban it. It’s not a particularly robust doctrine. The Supreme Court has held that pretty much everything else—including the Westboro Baptist Church’s protests—are not fighting words. The reason this doctrine is more-or-less abandoned is that it only gets used against incredibly unpopular people. And half the point of freedom of speech is that you can have unpopular viewpoints.