I think you're right that it's probably been happening for some time but may have started out small, maybe as just smuggling pot, then as they got away with that and became bolder and bolder, they gradually started to bring in the harder drugs, as that would be more money.
It seems like the security and scrutiny for the crew reboarding ( or boarding) a ship would be as tightly controlled as it is for the passengers. If a crew member can get aboard with quite a large haul of hard drugs, I think I would be questioning just how thorough the security actually is that's supposed to be monitoring the people coming and going, regardless of whether they are crew or passengers.
And too, once they get it on, do they, the crew use it to get high, or are they merely the middle man, so to speak, moving it from one place to the other for pick up?
I realize speculation is not always good but more than likely if it's happening on one cruise line, it's likely happening on some of the others too. Maybe they just haven't been caught yet.