The two NCL-Hawaii ships had hulls built in the U.S. then an act of Congress(!) allowed them to be finished in Germany and be flagged in the U.S., so it's not an exception easily granted.
3 ships were built as a result of the failed Project America.
https://en.wikipedia.org/wiki/Project_America
Pride of America
Port of registry:
United States,
Honolulu
Norwegian Sky, then Pride of Aloha, then Norwegian Sky again
Port of registry:
Bahamas (1999–2004)
United States (2004–2008)
Bahamas (2008–present)
Pride of Hawaii, now Norwegian Jade
Port of registry:
Honolulu,
United States (2006–2008)
Nassau Bahamas (2008–present)
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NCL's Involvement[edit]

The
Pride of America, a derived version of the Project America ships.
In late 2002,
Norwegian Cruise Line (NCL) contracted with
Northrop Grumman Ship Systems, the owner of Ingalls, to buy all of the materials, equipment and work performed on the two
Project America vessels. All materials were towed to
Germany and the first ship was completed at
Lloyd Werft Shipyard in
Bremerhaven. The ship was modified to increase its size, now 81,000
GT, and exterior and interior changes were made. This ship was named the
Pride of America and entered service in 2005.
The materials for the second
Project America ship were utilized in the construction of the
Pride of Hawaii (now named
Norwegian Jade), at
Meyer Werft Shipyard in
Papenburg.
Pride of Hawaii entered service in 2006 and is approximately 12,000
GT larger than
Pride of America. It is considered a
Jewel Class ship, which is a different class entirely than that of the
Pride of America.
Passenger Vessel Services Act[edit]
In exchange for its purchase of the
Project America materials, NCL lobbied heavily for an exception to the
Passenger Vessel Services Act, which prohibits foreign built ships from operating between U.S. ports without making a foreign stop in between. The exception was granted and three ships (the afore mentioned as well as
Pride of Aloha), effectively all foreign built, were allowed to fly the
U.S. flag. This exception allows NCL to have an exclusive right to operate Hawaiian itineraries that do not need to touch a foreign port. A caveat of this exception, however, requires NCL to staff these ships with a crew made up of predominantly U.S. citizens.