In a way you are missing the point, folks dont expect to pay what they pay back home for drinks as the ship is "Duty Free" or less and does not pay the same tax that a bar would back home.
So depending on the level of tax in the "registered" Country, not the sailing country,,ie usually Panama etc then tax laws are less, hence the ship registration to that port/country.
We have a scenario of the lines taking advantage of reduced tax laws on certain items,,so they register there. But they do not pass on the gain in tax including drink prices to the people that sail with them,,,they mark it up to make up for selling cheap cabins in the first place.
Dont believe me, then why when you look at the butt of any Carnival ship are they registered in Nassau or similar ports? Not Miami, as they would have to pay US tax.
I'm guessing the lines are using a major loophole here, they appear to be American to you or I, but not in their tax liability, unlike you or I.