No... actually, it's because one can fly SJU-MIA-LAX-LIH on one flight number (AA285). Although the plane lands twice enroute, the flight number doesn't change, so it only counts as one segment for mileage purposes, and presumably for the OWE as well.

On the return, there's no single flight number that goes from LIH to SJU (or from any other Hawaiian city, AFAIK), so the only possible flight combination violates the "BACKTRACK BETWEEN HAWAII AND THE CONTINENTAL U.S/CANADA NOT PERMITTED" rule.