@pastorlan, it doesn't matter why the people think it is a good idea, (whether to preserve their own temporal power, or because they actually believe God moght smite them - despite not smiting all the heathens in the next town who worship god differently) - i am making an argument for the evolution of ideas. That if an idea has some form of protection, it will out last an idea which does not.
And if only one 'god idea' can be held in each person's brain at a time, then the ones which die out allow the other ones feetile ground to grow. Now i know this is overly simplistic, i am ignoring lots of specific examples of the cultural importance of a lot of thigs related to religions. Like how Judaism managed to survive thousands of years after the second temple was destroyed... But i'm just saying that on average, all other things being the same, you can assume blasphemy laws would promote the survival of an idea.
@"In the world where gods are assumed to have power, the person who curses god is a threat to public well-being and needs to be stopped before he causes the gods to curse the nation." - that only works if people actually believed the gods would smite you - and yes, people did attribute (and to some extent still do) so-called 'acts of god' to a specific destroyer.
Infact i would argue this is the very basis of religion. That sometimes a drought would hit, and someone has a theory... So you go petition the local river spirit or whatever, and hope that next week or next rainey season or whatever things will be better (maybe after making a sacrifice). And if it doesn't work you sacrifice the guy who made the petition - cause he clearly did it wrong... Until some good weather happens along and you keep doing the thing you think you've proven works.
So to some extent, you seem to have a solid theory. Eventually after some urbanisation and a priestly class has gone full time, they have a pretty good gig. Instead of people coming to them when the rains fail, they tell people how to live, (and largely what animals to sacrifice in the temple) to ensure a successful or happy life. Wars off illness or bad luck... But these priests now have a vested interest in the population listening to them, because they are dependent on the surplus production of othwr workers to eat (even if they are eating the sacrificed remains of some animals - in more modern churches we're looking at sacrificing money to the church, seen in the extreme in some evangelical churches in the US today). Now maybe the early priests worried about competition, and maybe the same mids of circumstances as happened previously helped create new superstitions (like an earthquake hits your city, and now suddenly the preacher is blaming it on the unbelievers from the north...).
But it doesn't matter why the edict to ban blasphemy comes down. It only matter whether it is an effective tool for preserving a particular idea.
When Islam specifically talks about converting other by war, and executing those Muslims who turn their back on their faith, it is an example of using violence to protect the meme. Of course there are many effective strategies to protect your ideas. Judaism may have kept it in the family in a way which maintained the population relatively stable and not spreading too fast or diluting, but kept a very strong bond in those families (i don't know, i'm guessing here).
If christians going on crusades to protect the holy sites in Arabia were doing so for personal power, it doesn't matter. The personal reason don't come into it, the only question is whether it was effective at keeping the idea alive (and the we only have these successful ideas left...).
So pacifism may not be a very good idea, because it can't force others to put down their arms and if attacked it can't defend itself; yet it still has its adherents. It is not a dominant idea... And one culture can have many different, even conflictin ideas. Just as it can have many different religions sharing the same space... Anyway, i think i'm rambling, but it is late and we're talking at cross purposes.
It is possible that we are both taking complimentary descriptions of the same phenominon. Mine is more meta, yours is more practical. I believe there are multiple reasons to justify blasphemy, and they have probably all been used at one time or another. But if/when it is a successful strategy for preserving an idea (and i think it could sometimes backfire) my description of why blasphemy laws exist is an accurate one... (The laws themselves are ideas, made manifest and influencing thelives of their hosts... We think humans are great at manipulating our environment, ideas can change their hosts behaviour, get them into wars, or producing enough food to urbanise into cities... Maybe memes are the most powerful life form on the planet...)