When I travel to SYD, it is to go and see my parents who are in their 80s. If I were on that flight I would not want people coming off that cruise ship being on my flight. Kudos to the Untied crew for looking after the welfare of the other passengers and shame on United for allowing the cruiseline to book the returning pax on a regular flight.
There are a number of Australians who are trying to get home as it is expected Australia is going to close their borders (and is asking all non-citizens/residents, i.e. tourists to leave Australia). Everyone returning to Australia needs to go into quarantine for fourteen days but that is to protect the larger community. I do everything I can to isolate myself before traveling during these COVID19 times and I certainly do not want to be on a plane with people off a cruise known to have the COVID19 virus.
Those pax were lucky there was a hotel willing to take them - imagine the cleaning the housekeeping folks are going to need to do.