flag of convenience

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A foreign flag under which a merchant vessel is registered for purposes of reducing operating costs or avoiding government regulations.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun figuratively Registration of a ship in a country chosen for its low taxes, permissive regulations, and so on.
• noun literally The flag flown by a ship so registered (which flag then corresponds to the country of registration).