This photo was taken onboard Princess Cruises, in one of the bars, but majority of cruise lines have something similar in effect.
Since most cruise lines care much more about profit than anything else, bar staff tends to turn blind eye on this warning, if involved passengers keep it in check.
The logic behind it is clear.
Bar staff, bar supervisors, bar manager, hotel manager ... all that people earn more money if they sell more drinks.
Their income comes through tips and sale percentage.
Another thing, people come cruising in order to relax, and if they hear they could be imprisoned or fined for something they regularly do back home, they might decide to avoid cruise lines who follow the law to the letter. Granted, this is not the US law, since most of the time cruise ship will spend in international waters or foreign country, but sometimes lawyers can perform miracles.
At one hand, there is PR nightmare, and at the other - pressure to sell and profit no matter what.
Typically, it is a bar server or a bartender that gets thrown under the bus, if an incident occur and a matter gets to the court of law. Most of the time it doesn't.
Uninformed passengers are likely to take a free cruise as a compensation for finding their teenagers in onboard medical centers, diagnosed with alcohol poisoning.
"A person who provides alcoholic beverages to a person under 21 years of age, if convicted under AS 04.16.051,
could be imprisoned for up to 5 years and fined up to $50.000."
