I didn't read the book but it seems like a hand-wavy justification for having kids kill each other and the movie is better for not having it.
It is actually a good one from a storytelling perspective. The entire point of the story is that kids kill each other for no good reason, so give a reason that is not good. Not giving a reason would have the reader wonder about the "why", which is not the point.
I didn't get that point at all, I thought it was more about how elders in society exploit the young, which is doubly true in a country like Japan with its poor birth rates
It is actually a good one from a storytelling perspective. The entire point of the story is that kids kill each other for no good reason, so give a reason that is not good. Not giving a reason would have the reader wonder about the "why", which is not the point.