Certain powerful magics or future technologies can have such wide-ranging and devastating effects that worldbuilders may wish to limit their use. Avoiding these elements because of cultural taboo is one approach, and one extreme manifestation of that is simply making the use of a fictional element a war crime within the fictional setting. If worldbuilders do not wish to deal with the headaches of orbital bombardment in their science fiction worlds, they may make it a war crime to turn planets into glass, thus limiting its’ prevalence. If necromancy is theoretically possible but doesn’t fit the narrative, it may be a war crime to raise the dead for battle.