I can't believe someone brought up NJS, and forgot to mention 3rensho.

There's nothing special about NJS frames over a regular track frame, they just have hipster tax on them. Well, they might be painted better and be heavier, but that's about it.

I remember in 2007 or 2008, bridgestones RB1, RB2, RB3 were going for something like $300-500 because of hipster taxes.

NJS is simply a standard to keep competition about the rider than the bike he's riding while racing in keirin.
not much different from how UCI dictates what a bike is for their events. It's actually kinda stupid to buy NJS parts when there are better parts on the market, from a functionality perspective.