I agree. It's approximately 2300 miles from San Francisco to Hilo.
The SSUS at 30 knots, its normal cruising speed, could travel that distance in 67 hours, slightly less than three days. If the SSUS left San Francisco Sunday night at 6 pm, it could be in Hilo by 1 pm Wednesday afternoon. Tack on NCL's common 7 day itineary to that, it could be in Honolulu as the last stop for those wishing to fly back Thursday morning a week later, for a total of 10 days. For those sailing back, it could be back in San Francisco Monday morning, for a grand total of 14 days.
You could market the same cruise either way.
Sail both ways, or just one way and fly the other. That's something you couldn't do with a cruise ship with an International Flag. They would have to visit a remote foreign port (Fanning Island being the closest) to not have to do a round trip cruise. Cruise ships that do one way Hawaiian cruises start or end their itineraries from Mexico or Canada, not from a US port. Possible money earning itineraries exist with a true American flag ship.