The rule is if more than 3% of a ship is confirmed to have the virus, the cruiseline is required to report the incident to the CDC. This is not to take any punitive measures, but because it allows the CDC to inspect and study the outbreaks. It allows them to study the virus in a contained environment. This virus is very common in the US and is often carried aboard by someone. The incidents of outbreaks are becoming less and less as the cruiselines have instituted measures of sanitation and quarantine to contain the virus.
I don't have any problem with a history of norovirus on any ship. I wash my hands and take measures to protect myself, and I know the cruiselines sanitize and clean their ships after every cruise.