It made sense initially, but now a quarantine might not make sense.
See this Washington Post article, Cruise ship coronavirus infections double, exceeding the total for any country but China.
When the ship docked in Japan, it was quarantined for 14 days to be sure only healthy people disembarked. But now, the only people disembarking are those who are sick and being taken to hospitals. Ships are places of close quarters and even though they are implementing social distancing with a variety of measures, people continue to get sick. Now, are they getting sick from previous contact, or are they being newly infected?
Because of all the uncertainty, they are likely putting "all options" back on the table. A shipboard mutiny is possible.