[AHASIC]

Nah, this is a bunch of crap. I worked on a carnival ship until the pandemic and I don't know where you pulled this answer out of, but it plain old wrong.

The reason they can't leave is simply the American authorities won't let them leave the ship in order to prevent them from staying in the U.S and potentially finding a way to get a job there (legal or illegal). There wasn't a person on board who couldn't leave the ship because of documentation in normal circumstances, but with the virus happening, Carnival shit their pants and were blocked from having anyone leave for the aforementioned reason.

[FpUser]

Which proves once more that the governments do not really give a flying fuck about people and are willing to keep them in jail like conditions without those people committing any offence.

[AHASIC]

I believe that the US citizens who working on board could leave, it's the NON-US citizens that the US didn't give a shit about. Hell, keeping the crew onboard meant that the crew will spend their money onboard for necessities like water, cigarettes, laundry and drinks, so that they could make a bank out of bad situation.