Seems some of the companies just bootleg them through. Unless things have changed since I bought my pack and 2 canisters?? the canisters are not approved by the FAA for flight in a commercial jet over U.S. soil. Completely legal in Canada though. I believe it's just a matter of the company sending the canisters through a U.S. certification proccess. Regardless, it is a pain to get them up here as they are supposed to be ground transportation only.
That's what I was told at least.