Tyranny does not come as a thief in the night, but openly. The killing, by two radicalised islamic extremists, of a British soldier on a London street has brought the usual calls to suspend the rule of law. A BBC interview with one radical preacher prompted Home Secretary Theresa May to ask what the state broadcaster thought it was doing by broadcasting such a thing, and Baroness Warsi, formerly a prominent Muslim minister, joined the chorus. The newspapers reported that a ban on radical clerics being covered on the airwaves is being actively discussed.

If so, it won't work. Those with long memories will remember how the UK government tried to prevent the broadcast media carrying interviews with radical politicians from Northern Ireland. That came about after a particularly sickening interview with Sinn Fein leader Gerry Adams on the back of yet another Irish Republican Army murder. The law was duly passed, but the media simply used actors to dub the words of their interviewees, or put the text on screen and had the newsreader read it out.

Frankly, I don't want to hear extremists justifying murder on UK radio and television. It can be pretty revolting, not to mention deeply upsetting for bereaved relatives. But we have – or had – a rule that people should be free to express their opinions, no matter what the rest of us think about them. We have that rule because we believe that, although it may be abused on occasion, and although it may give air to views that might prove damaging, in the long run it is better to have ideas openly expressed and debated. If ideas are good, they will win that debate. If they are bad, they will not.

There is a limit, though, as the libertarian philosopher John Stuart Mill pointed out over a century ago. We do not allow people to say or do things that could cause real damage to others. We do not allow people to shout 'Fire!' in a crowded theatre, and we do not allow them to incite violence. The line between condoning a murder and inciting violence might be a thin one for the broadcasters to tread. But we owe it to the rule of law to try to maintain that line.