I just shot a quick email to the NPS to double check. Realizing the NPS and Forest Service are two different entities Forest Service personnel in the Gallatin and Shoshone are REQUIRED to carry it when ever they leave their vehicles. The spray has been advertized as a Non lethal way to make bears change their minds about messing with humans. I know that Wynn's(before it when out of business) carried it after the fatal attck on Little River.
I've posted recommending carrying the Spray for years on various bulletin boards, and never had anyone remark that it was illegal. Counter assault Bear Spray has propellent shelf life of *2 years, I been carrying the cans that I obtained in Cody, Wyo. ever since having throwing my sleeping bag out alongside my car in a N. F. campground in S. Ky and waking up to the growls of someone's Fido and his friends.