The distinction seems to be based on the level of disability in the survivors. Dead people are not a burden after the war is over, but maimed people are, and a

The distinction seems to be based on the level of disability in the survivors. Dead people are not a burden after the war is over, but maimed people are, and a huge part of that is blindness. Old-style chemical weapons tended to blind people. Post-war civilian life doesn't work so well if a significant chunk of the population is blind.