While the NHS designation is definitely criteria for classifying a road as primary or higher, this might be a rare exception, as the NHS-designated segment of Casino Drive is only a spur leading to some casinos rather than a major transport hub like an airport or seaport. In fact, I'm not even sure why the FHWA designated this as an NHS route, but I guess it might have to do with some planned development around Laughlin that was scrapped or postponed indefinitely (which there is plenty examples of in the area) or maybe the generating station. But on its own merits, Casino Drive isn't used as a through route as much as Bruce Woodbury is, and Thomas Edison Drive has a quite low AADT.