Why would they do this for the flu?
You want to go on a cruise and get the flu?
A problem all cruise ships have is the crews rotate from all countries around the world and some countries do not handle screening of those working people for corona virus as well as others do.
So the virus could be transmitted to healthy passengers from a crewmember. -Tom
In a year where the seasonal flu had an r0 over 2 and a mortality rate of 2%, they’d do the same thing for flu.