In their culture human life is not so important as the book.
Now what is it that we are not being told?
Same thing as always. On any given day, a murder as horrid as this happens. That it will happen because someone might have seen an ankle, or they dare teach their daughters, or they didn't give their 8 year old as a bride, etc. That while we view women and children as the future, to them they are a burden. That they will forever care more about burning a Koran than if an entire village was wiped out.
That we are dealing with a people completely and utterly divorced from any humanity we would recognize.