Adam Curtis, in an intriguing post about cruise ships, explains why so many ships sail under flags of convenience:
All this happens because of The Flag of Convenience. It was an idea that the Americans came up with in the early days of the second world war to allow them to send help to Britain. Roosevelt was worried that Hitler might declare war on the US – so a law was passed that allowed American ships to be registered either in Panama or in Liberia.
The Flag of Convenience was born out of altruism, but it is now used for purely selfish reasons. Many of the cruise companies register their ships in countries such as Panama and Liberia, this mean they do not have to pay corporate taxes in the US and aren’t bound by many labour regulations.