We're allowed to import carbureted minis without airbags because people would be crazy to daily drive them, and they make nice collectors pieces while polluting minimally driven on weekends.
Add fuel injection and an airbag, still in a lightweight chassis, and someone will daily drive it, depriving BMW of NA money from selling a new mini.
-- so goes their logic, I'm not defending it. Mercedes of USA is pretty much the culprit behind this whole thing-- German Merecdes are not quite the same status symbol over there, something MBofUSA invested heavily in promoting. They don't want their knees knocked out from under them. The other euro importers are riding on MB's coattails, also pretty happy.