The friends of freedom should not make exceptions because freedom's enemies never do. Admittedly, the trailer for Innocence of Muslims (one of its many titles) makes the temptation to allow just one exception close to overwhelming. It advertises an amateur and adolescent piece of religious propaganda that depicts Muhammad as a violent and lascivious fool. Copts probably made it. As there is no great difference between Christian and Islamist extremists, why not intervene in this clash of fundamentalisms and stop one sect inciting another sect to violence?

Even before mobs attacked the US embassy in Cairo, its diplomats felt the urge to abandon basic principles. "We firmly reject the actions by those who abuse the universal right of free speech to hurt the religious beliefs of others," they said. Hillary Clinton was hardly more robust. "The United States deplores any intentional effort to denigrate the religious beliefs of others. Our commitment to religious tolerance goes back to the very beginning of our nation." It was a little too late in the day before she recalled America had other commitments going back to its founding, and muttered for all that America still does "not stop citizens from expressing their views, no matter how distasteful".

European states, with all their counter-productive restrictions on freedom of speech – and yes, thank you, I include laws against Holocaust denial, denial of the Armenian genocide and all the other prohibitions of hatred that litter the statute books – would find a way to ban the film and arrest the film-makers. The British police would use public order and breach of peace laws. The wistful tone of the Obama administration make one suspect that it wished the US constitution did not prevent it following suit

Innocence of Muslims is one of the hardest cases for liberals I've come across. But even this tawdry piece of work raises problems for the proponents of censorship. The first is a problem with language. Mount a critique of Islamist religious fanaticism, and it is only a matter of time before you find that defenders of religious reaction have hijacked liberal language. You are an "orientalist", they say, an "Islamophobe", "neo-colonialist" or "neocon". (The prefix "neo-" has become a synonym for "evil". The reader need only see a "neo-" to know that no good will follow.)

The joke of it is that defenders of censorship represent "orientalism" at its most patronising. They see the world's Muslims as an undifferentiated and infantile mass. The smallest provocation – a cartoon in a Jutland newspaper, a trailer for a nasty but obscure film – is enough to turn them into a raging mass of bearded men who bellow curses as they fire their Kalashnikovs. They take no account of those in Libya, Egypt and Iran who want nothing to do with clerical violence. As seriously, they do not understand that "offences against Islam" are manufactured by extremists, who must keep their supporters in a state of violent rage or see their power wane.

The murder of US diplomats was not carried out spontaneously, but by a jihadist militia that wanted to kill Americans on the 9/11 anniversary. In Egypt, the controversy over the Coptic film was created by Al-Nas, a Salafi channel dedicated to promoting militant Islam. These crises are political events, in other words. Their promoters must create the poisonous atmosphere in which they thrive. Does anyone doubt that if the Muhammad film had never been made, they would not have found another target for their fury? Has everyone forgotten that their targets have included men and women liberals have a duty to defend? The same people who scream today, applauded the murder of Salman Tasser for protesting against the execution of Pakistani "blasphemers" who "insulted" Islam. They hoped for the murder of Ayaan Hirsi Ali, because she tried to stand up for the right of immigrant women to resist religious oppression in Europe.

Then of course there is the case of The Satanic Verses. Salman Rushdie has chosen this week to publish his autobiography. I would have said that the timing was perfect from his publisher's point of view, except that so many other weeks would have revealed how the violence caused by Ayatollah Khomeini's attempt to suppress The Satanic Verses in 1989 and murder all those associated with it never passed. Readers who were around at the time will remember that a desperate Rushdie tried to appease his persecutors by issuing an abject apology. He learned that there are forces you cannot appease, when the Islamists laughed and carried on with the terror campaign. It is a lesson we would do well to remember.

To bring the story up to date we now have before us the example of Channel 4's documentary on the origins of Islam. It was everything that the Muhammad trailer was not. Tom Holland presented a thoughtful and balanced film on the arguments among historians about whether the armies that exploded out of Arabia to conquer the Persian empire and much of the Byzantine empire were Muslim, or whether Islam came later. His documentary was public service television at its most scrupulous. I speak from experience when I say that he has no hatred of religion. The last time I met him was at a debate where he argued for and I argued against a motion that religion was a force for good in the world.

Nevertheless, Holland and Channel 4 had the integrity to break a taboo more frightened broadcasters are too cowardly to challenge. They aired doubts about Islam's founding myths, and the predictable fulminations followed. The Ramadhan Foundation and Islamic Education and Research Academy attacked them with dangerous abandon. Channel 4 had distorted "our faith and history", they said. The programme was "prejudiced" and "ignorant". Their denunciations are all over the web, and could be picked up in Iran or Egypt or indeed Bradford or Birmingham and used as an excuse to attack British interests. Does that possibility mean Channel 4 should have suppressed the programme, and that Britain should submit to a de facto blasphemy law?

Even in the hardest of cases, the old arguments against censorship remain the best. The makers of Innocence of Muslims have reactionary religious prejudices and probably reactionary racial prejudices too. Reactionaries are not hard to beat in open debate. If you can't beat them without calling for the cops or reaching for a gun, you should get out of the debating business and make way for someone who can.