WASHINGTON—The decision by Comedy Central, the television channel owned by MTV Networks, to censor an episode of “South Park” that alluded provocatively to the Prophet Muhammad illustrates once again the fear terrorists acting on behalf of Islam have instilled in the Western media.

“South Park,” the animated sitcom featuring four children characters in a fictional town, takes on all manner of institutions and established truths with crude language. It recently celebrated its 200th episode with a couple of programs that brought back old characters. The first one had Muhammad dressed in a bear suit, prompting a small Muslim group in United States to fire off death threats over the Internet against Trey Parker and Matt Stone, creators and directors of the series. As a result, Comedy Central censored parts of the second program relating to the prophet with image blocks and bleeps. The creature disguised as a bear became Santa Claus.

In one post, Zachary Adam Chesser, a university dropout who has written rubbish on Twitter and on RevolutionMuslim.com, a website for the 12-member organization behind the threats, asked Allah to burn Stone and Parker in hell; in another he predicted that they “will probably wind up like Theo van Gogh.” The FBI and the New York police are investigating.

It is not difficult to fathom Comedy Central’s actions. The threats and violence against artists who have referenced Muhammad in unorthodox fashion or have simply depicted him are well-known. Among examples: Salman Rushdie, the Indian writer targeted by an Iranian fatwa; van Gogh, the Dutch filmmaker murdered in 2004 for his documentary “Submission”; the Danish newspaper Jyllands-Posten that drew deadly protests when it printed 12 cartoons of the prophet in 2006; and Dutch politician Geert Wilders, viciously threatened for his film “Fitna.”

But precisely because of these antecedents, the decision to censor the episode of “South Park” is especially troubling. An earlier episode in 2006 that satirized the way in which the Western media cower to blackmail by fanatics who do not tolerate images of the prophet had already been censored; later, the president of MTV Networks, a division of Viacom, regretted publicly caving in to political and commercial pressure. This time, the bear suit in the first of the two episodes implied a mockery not of Muhammad but of the Western media, which makes no apologies for trashing other spiritual icons, including Jesus, Moses and Buddha—all of whom were ridiculed in that episode, by the way—and has accepted the Muhammad taboo, avoiding all depiction of the prophet. One of the child characters in the show, incidentally, often mocks Jews even though one of the directors is Jewish.

Comedy Central would not have been right to censor the first episode, but it made things worse by showing the first uncensored and then censoring the second one in response to the threats. Any time fanatics carry the day against free expression, something essential is lost.

MTV Networks’ right to broadcast what it wants is beyond question. In fact, it deserves praise for airing “South Park” regularly despite pressure from organizations that denounce its influence on children and from moralists for whom established beliefs are untouchable. MTV Networks has also brought refreshing humor to the world of politics in other shows on Comedy Central. But all this only makes the decision to bow to the threats all the more meaningful.

The libertarian vein of “South Park,” which certainly can be offensive and unpleasant, hails from the great Western tradition that goes from the Greeks to Voltaire, and from the Enlightenment to the modern heroes who battle totalitarianism in Zimbabwe, Iran, Burma, North Korea and Cuba today. One does not need to admire its content to appreciate its moral superiority over those who forced the self-censorship on MTV Networks and the need to stand behind creative freedom.

Sometimes, images of Muhammad do not cause any reaction. A 2001 episode of “South Park” featuring the prophet went unnoticed. But other times, when hatred is whipped up by cunning radicals, they can be a pretext for ugly stuff. This is something free societies are going to have to live with for a long time—and learn to address appropriately. Self-censorship cannot possibly be the answer.