The Geneva convention doesn’t apply to your own citizens, just to war. The US made sure they had that exception in hand
When they formally banned tear gasses (including this specific one) in 1993, since the Geneva convention only had a loose definition, the US again reserved an exception for domestic use.
So yes. The US has promised they wouldn’t use tear gas against any other country because that’s inhumane and too horrible, regardless of the reason for war. But it’s fine if they use it on protestors. That’s totally fine, they shouldn’t have gotten too rowdy.