I'd say that for the majority of people it has very little (if anything) to do with either.

1. Keirin frames are (relatively) difficult to come by, thus they carry a certain cache.
2. The current surge of interest comes from a generation that grew up on Robotech, Transformers, Astro Boy, and a host of other cultural products imported from Japan. These products were usually of far superior quality when compared to their domestic couterparts; it's not a stretch to think that one's interest in the products of Japanese cultural production would continue as one grows older and develops new interests.
3. There is a healthy dose of good ol' fashioned fetishizing of the "other" going on. eg, there are quite a few Japanese collectors who are crazy about vintage US guitars and other by-products of american "surf" culture. People tend to be drawn to things that seem exotic and arenot a part of their own cultures. Remember when upper crust Britons would pay millions for a single tulip?
4. The NJS provides reassurance in terms of quality of construction. It is difficult for the layperson to pick up two track frames, look at them side-by-side, and determine which is 'better' in terms of materials and construction. While you might be able to figure out that a Schwinn Paramount is a better bike than a Schwinn Madison, it's a little more difficult to quantify why a Columbus Thron Cinelli Olympic is 'better' than a Columbus Thron DeBernardi (or at least costs twice as much). The NJS stamp, to a certain extent, says "this is a professional quality bicycle made to adhere to exacting standards." Whether you understand the 'exacting standards' or not, that stamp offers assurance of a base level of quality. Bikes of other nationalities, simply because their regulatory bodies don't seem to care about little stamps, cannot offer this kind of assurance unless the person purchasing the bike is familiar with the particulars of the model, builder, marque, company history, etc. Simply put, even if I have never heard of Peleton or 3Rensho or Yanagisawa or whatever, the NJS stamp tells me that the bike is made to a professional standard. If I've never heard of Casati, how can I tell if I'm looking at a top-quality, master-built bike or a club rental built for those just learning?
5. Don't discount the power of the zeitgeist and one-upsmanship. Track bikes are cool. The rarer and more exotic your track bike, the cooler it is.

Given that I don't currently own a keirin frame, nor no I have any interest in actively seeking one out, others may have more to offer to this discussion than I.

m.