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I don't know why this should be illegal and I rather assume that the cruise line isn't willing to make an exception here because of the probably higher expenditure of disembarking a single passenger or two. The cruise line has to comply with lots of regulations including the infamous ISPS code.
What would be illegal, however, is a disembarkation prior to the ship's visit to Kiribati, as foreign (i.e. non US-flagged) ships have to comply with an ancient US law from the 19th century called the Passenger Services Act which prevents foreign-flagged ships from carrying passengers between US ports. If traveling between US ports (e.g. on a cruise roundtrip from Hawaii) a foreign-flagged ship must visit a foreign port inbetween.