The law was just brought in so that the law matches the constitution (and this was acknowledged by those that passed it). It's written in such a way no one will ever be prosecuted. The only alternative would be a referendum to change the constitution, and what politician wants to seem like they support blasphemy? As well as that, when the law was passed the next referendum was for the Lisbon treaty, and the government wouldn't have wanted to hold the two referendums together (as it'd make the Christian Right more likely to vote, and so possibly sink the Lisbon referendum).

And not just America. We also have it in Ireland (imaginatively named CTYI - although in our case the 'T' stands for 'Smart').