I see several probs. First they didn't get a fare quote. Then when they went to the police station, they didn't act accordingly. Now they are getting an extended stay on Antiqua. This is why people stick with the ship's excursions. And everything I've read says to get a fare quote BEFORE the ride.
Presumably they were okay with a $50 fare that ended at the beach so they should have paid that and gotten another taxi back to the ship.
It seems the main probs happened at the police station. It does sound like the passengers are at fault in the melee with police. Hopefully they'll get a fair trial, but regardless, they are not able to leave the island anytime soon.