Put bluntly, they can use rifles, shells, knives and swords to slaughter each other - but absolutely no sarin.

I think this is called The Logic of War. Or something.

It's perfectly fine to kill enemies, as long as you do it with sharp metal, projectiles, and high explosives.

Chemicals, bio-weapons, and nukes are the wrong side of the red line.

Presumably this makes sense to someone out there.

I suppose there's an Argument from Escalation, which is that once someone uses them everyone will and really, really bad things will happen.

Which makes a kind of sense, and is probably true, up to a point.

But still - maybe it's the killing that's the real problem, not the weapons it's being done with.