In a statement, U.S. Customs and Border Protection (CBP) announced the reasons why, beginning in 2016, changes concerning Cruises to Nowhere will be made so these short sailings into international waters without calling at any ports will no longer be allowed.
As explained in a federal court opinion of 2014, it had been the CBP longstanding position that D-1 visa holders were not eligible to work as crewmembers on Cruises to Nowhere. Under the INA, D-1 visa holders were eligible to serve as crewmembers only if they intended to land (in the U.S.) temporarily and solely in the pursuit of calling as crewmen and to depart from the U.S. with the vessel.
Although these cruises do enter international waters, Customs and Border Protection maintains that passengers (which includes crewmembers) aboard don't actually depart from the United States because "they do not land in a foreign port or territory".
This means that the only way for Cruises to Nowhere to continue is for the lines to employ an entirely American crew for each vessel doing a CTN. Otherwise, each short sailing has to include one foreign port call (at least) before the ship is allowed to return to the United States with crewmembers who are not American citizens/lawful permanent residents authorized to work in the U.S.
Currently, the only ocean-going ship hiring an entirely American crew is NCL's Pride of America. By law, it is required to employ an American crew as it's a U.S.-flagged ship based in Hawaii and sailing only Hawaiian Islands itineraries.