At least in my experience, it's not just that imperials will get tear gassed, it's that they're not psychologically, or equipmentologicaly, prepared to be teargassed. Americans show up to stand in a park, hold their sign, and go home. They're not showing up to pick a fight and they don't really know how to incorporate the idea that the police will violently suppress them just for "peaceful assembly" in to their world view. They're domesticated.