It probably goes back to ancient times, and some idea that war can be honourable but only if fought in a particular way. I know that the Romans, early in their history, had a passionate -- and at times a fatal -- distrust of anything that was "underhand": spying, scouting, even ambush.
Presumably, it's the same thing carried forward: that there is a right way to slaughter your enemies, and anything that deviates from that is dishonourable and deserves the highest of punishment.