The regulation regarding harassing agents is not about treating service members a certain way, it’s about avoiding escalation to lethal chemical warfare. The risk of a civilian escalating to mustard gas or nerve gas after getting hit with CS is very low.
A better example for what you’re talking about would be hollow point bullets. 100% war crime to use because of its effects on the body, never saw one the whole time I was in the Army, yet it’s the type of bullet used by police nearly everywhere. It’s also what most home defense ammunition is.
Yeah, fair enough, it would be odd for immediate retaliation. However, let me present another scenario. Imagine harassing agents are permitted to be used in war. How long do you think it would take the war industry to develop significantly worse harassing agents than CS, and how much lawyering would end up happening to argue a certain chemical weapon is actually a harassing agent and not something that’s banned? Now that retaliation scenario isn’t that far out of the picture. Force A gets hit with some super harassing agent bordering (or actually) lethal chemical weapons, force B retaliates with mustard gas.
It’s easier to just ban them all.