This is hardly the first case of Abrahamic religions not making sense. If you going in expecting self-consistency and logic, you are going to have a bad time...
Anyway, I don't think the GP is entirely correct. Sure, perhaps to a certain extent religious people react negatively to the blasphemy of outsiders because it is the dogma of their religion to do so. I think the greater cause however is that all people, even religious people, tend to be rather rational. When people hear others poking fun of their religion they become upset, not because they "know" they are right (and most would say without hesitation that they do), but because somewhere deep down inside a part of them considers the possibility that the blasphemer just might be right. Blasphemy tickles those deep-seated insecurities. As rational as people are, they really don't like being wrong and made to look a fool.
I still think you're projecting your cultural context onto people with a different culture. If it was all about insecurity they'd react the way Christians reacted to "Piss Christ". But they don't, do they?
Anyway, I don't think the GP is entirely correct. Sure, perhaps to a certain extent religious people react negatively to the blasphemy of outsiders because it is the dogma of their religion to do so. I think the greater cause however is that all people, even religious people, tend to be rather rational. When people hear others poking fun of their religion they become upset, not because they "know" they are right (and most would say without hesitation that they do), but because somewhere deep down inside a part of them considers the possibility that the blasphemer just might be right. Blasphemy tickles those deep-seated insecurities. As rational as people are, they really don't like being wrong and made to look a fool.