I've had US troops use CS on me lots of times.
The worst was while in the back of a deuce & a half with the cover on, but open in the rear. Two CS grenades were thrown into that opening from a passing OPFOR jeep. It was only a FTX, but the CS was real. I saw them coming and managed to get masked quickly so I only cried a little. Two squads were in the truck, most got dosed much worse than me.
They can use it in training, but they cannot use it in actual fighting. How would Fallujah have gone had we been able to gas some of those buildings Marines were forced to assault?