The U.S. Passenger Services Act makes it illegal for foreign-flagged ships to carry passengers from one U.S. port to another U.S. port for the purpose of debarkation. Thus you cannot legally go from Los Angeles to any port in Hawaii and then leave the ship. The PSA is an old law meant to protect U.S. flagged shipping, but it is still in effect and enforced.
The PSA is also the reason that 15-night cruise has to stop in Ensenada, Mexico, prior to returning to Los Angeles. It is illegal for the cruise to go to Hawaii from Los Angeles without stopping in some other country prior to the return.