Air travelers from any country (U.S. residents or not) are covered by an international treaty that allows them to bring in controlled substances for personal use. Customs has a lot of discretion, though, in deciding what they'll let them bring in and how much. You're right that the "50" law applies only to residents returning at land borders. Another law applies to everyone else coming in to the U.S. so what you said is wrong. I'll look it up in the USC and CFR if i have to.