I'm certainly no expert, but the ships, in the most simplistic terms, are individual corporations. Based the Norwegian model, it would take an act of Congress to allow, say, Carnival, to register it's ships in the US anyway, since one requirement for that is substantial, if not total, US construction of the ship. The Pride of America was mostly built in Mississippi and completed in Germany, while the Pride of Hawaii was built in Germany with parts from Mississippi (and this ship was since been foreign flagged and re-named the Norwegian Jade).

For sure the cruise lines hate talking about wages, flags of convenience and stuff like that.