More on that Carnival Cruise bust…

Scott comes through for me (as always) with the full details of the Jam Cruise 3 drug bust (including, bizarrely, photos of all those arrested and video of the news coverage).
“Jam Cruise 3” Lands Twelve in Jail Before Ship Ever Leaves The Dock

The scenario was the same, over and over. As more than 1,200 passengers passed through the Jaxport terminal, they stopped for a photo, then proceeded on to Customs for a pre-boarding inspection.
Over a span of six hours, a dozen of those passengers were ‘outed’ by a proficient U.S. Customs and Border Protection drug dog named Megan.

So these were all people in the U.S., boarding a boat in Jacksonville, Florida and going through U.S. customs.
Now I’m trying to understand this. Isn’t it the job of Customs to prevent things from coming in to the United States? Yes, I know that the cruise goes outside our national waters, so Customs gets involved, but shouldn’t that be when the boat returns?
Is this at all an appropriate use of Customs personnel? We don’t have the resources to check more than a small percent of the cargo containers coming in to the country (any of which could contain terrorist weapons). But we can have Customs personnel checking for cruise passengers with some pot hidden in their crotch?
That’s criminal use of our homeland security forces.