Oh fuck off!
Why is it? Because the faithful made up that rule so that people couldn’t criticise them. This isn’t an argument for faith, it’s a (lame) reason to stop talking about it. The late lamented Douglas Adams (God rest his soul ;)) had some useful stuff to say on this matter, and he’s funnier than me so I’ll hand this post over to him:
“Now, the invention of the scientific method and science is, I'm sure we'll all agree, the most powerful intellectual idea, the most powerful framework for thinking and investigating and understanding and challenging the world around us that there is, and that it rests on the premise that any idea is there to be attacked and if it withstands the attack then it lives to fight another day and if it doesn't withstand the attack then down it goes. Religion doesn't seem to work like that; it has certain ideas at the heart of it which we call sacred or holy or whatever. That's an idea we're so familiar with, whether we subscribe to it or not, that it's kind of odd to think what it actually means, because really what it means is 'Here is an idea or a notion that you're not allowed to say anything bad about; you're just not. Why not? — because you're not!' If somebody votes for a party that you don't agree with, you're free to argue about it as much as you like; everybody will have an argument but nobody feels aggrieved by it. If somebody thinks taxes should go up or down you are free to have an argument about it, but on the other hand if somebody says 'I mustn't move a light switch on a Saturday', you say, 'Fine, I respect that'.
The odd thing is, even as I am saying that I am thinking 'Is there an Orthodox Jew here who is going to be offended by the fact that I just said that?' but I wouldn't have thought 'Maybe there's somebody from the left wing or somebody from the right wing or somebody who subscribes to this view or the other in economics' when I was making the other points. I just think 'Fine, we have different opinions'. But, the moment I say something that has something to do with somebody's (I'm going to stick my neck out here and say irrational) beliefs, then we all become terribly protective and terribly defensive and say 'No, we don't attack that; that's an irrational belief but no, we respect it'.
It's rather like, if you think back in terms of animal evolution, an animal that's grown an incredible carapace around it, such as a tortoise—that's a great survival strategy because nothing can get through it; or maybe like a poisonous fish that nothing will come close to, which therefore thrives by keeping away any challenges to what it is it is. In the case of an idea, if we think 'Here is an idea that is protected by holiness or sanctity', what does it mean? Why should it be that it's perfectly legitimate to support the Labour party or the Conservative party, Republicans or Democrats, this model of economics versus that, Macintosh instead of Windows, but to have an opinion about how the Universe began, about who created the Universe, no, that's holy? What does that mean? Why do we ring-fence that for any other reason other than that we've just got used to doing so? There's no other reason at all, it's just one of those things that crept into being and once that loop gets going it's very, very powerful. So, we are used to not challenging religious ideas but it's very interesting how much of a furore Richard [Dawkins] creates when he does it! Everybody gets absolutely frantic about it because you're not allowed to say these things. Yet when you look at it rationally there is no reason why those ideas shouldn't be as open to debate as any other, except that we have agreed somehow between us that they shouldn't be.”