Thinking more practically though. Why wouldn't there be "narco drones", with drone technology becoming so ubiquitous and cheap? And what would their operators care about airspace restrictions? The practical ones, as in "not get sucked into a jet engine or damage a wing and cause a plane crash"?
According to the CBP they seize about 50,000 lbs of drugs at the border each month which is about 22 tonnes of drugs, and that's what gets seized, not the amount that makes it through. So Drones today probably don't carry enough weight for far enough to make a big impact on the amount of drugs you can bring into the country. So it probably happens, but to do it at a scale where it's genuinely contributing to the total volume you'd need dozens of drones doing dozens of trips a day to be getting up to volumes that people would notice, and people would probably notice the drones first, and the drones are probably much more expensive than desperate people.