Since the US military does not classify WP as a chemical weapon (nor any other military in the world I know of), I'd be curious as to what they were thinking.
If you want to be really technical, a pointy stick with the end lit on fire is a chemical weapon. But no one in their right mind would put such a thing in the same category as mustard gas and VX nerve agent.
I'm not unsympathetic to your general point, but I think you're speaking towards issues far removed from WP. If it comes down to letting terrorists kill me or shooting WP rounds at them, I'll shoot them. Frankly, I think you would too.