Why don't I have a custom title by now?!
Random aside: Can't Allah take care of himself? I admit I am no theologian, but I don't get the whole mentality of wanting to kill people because they insulted your religion. If you're right, and your God is real, he not only has the power to punish them, but the power to punish them for all eternity. Why would you feel the need to take matters into your own hands when your God can handle it?
I keep thinking this is a stupid question, but I don't really have an answer, either. Even when I considered myself religious, this concept never made sense to me. I mean, the Bible was full of stories about God punishing nonbelievers, and because of that, I never felt it was my job to deal with them. It's like, the Bible makes it clear that bad people will get what's coming to them even if I do nothing. Why not just be content with that?
This is because you look at religion as if it is a question of faith, spirituality, and mysticism. But it's not. Or at least, not only. It's also a question of social control. That is the important part.
Now, what happens when one spreads blasphemes and heresies? Best case scenario, lightning comes down from the sky and strikes him down, convincing every witness that God punished the miscreant, and you get full attendance at the place of worship for years to come. But let's be honest, this is rather unlikely. In most cases, what happens is pretty much nothing, exactly as if the entire concept of God was a big lie, and some of the crowd listening to the ranting offender will be convinced. The fear of God decreases in the population, and that means that people escape your control, start behaving in ways you don't like, and worse of all, stop paying their tithe. I think we can all agree that this is pretty much unforgivable.
So, you've got to take matters in your own hands, and claim to be acting as the instrument of God. It's not you murdering somebody, it's God acting through you smiting an infidel. Much better this way. So the heretical and blasphemous ideas will not corrupt innocent souls and people will be afraid of God -- or at least, afraid of you -- and behave like God dictates through your mouth. Coincidentally, God has exactly the same ideas you do about how people should behave, but it's to be expected; it's why you were chosen in the first place.
So that's your answer. If you have an individualist view of religion, if you believe that it's up to everyone to go on a mystical quest of discovery so as to realize that yes, they have Faith in God; then indeed you are not going to care. Your faith is your faith, and cannot be marred by what other people think and say. But if your view of religion is one of group, of clan, then it'll be important for you to make sure other people think in the right way. You don't have time to waste waiting for them to walk through the path all the way; so you're just going to give them the cheat-sheet ("repeat after me: God hates fags! God hates fags! God hates fags! Okay, your catechism is over; now you know the basics and fundamentals, the rest is just window dressings.") and you're going to make them they don't deviate from the path you've given them. What matters isn't what you think, it's what other people think.