Edit: This is how it's legal.
It was brought in legally while Motorex was still a thing. This is one of very few cars which were actually emissions certified with cats and what not. When Motorex was shut down, the cars which had already been brought in and sold were allowed to stay in the US as bond-released, fully legal GTRs. Because they were already bond-released, they were not able to be seized. There are very very few Motorex cars in existence (under 50 IIRC), and very few of those were actually emissions certified. I know of someone with an ex-Motorex R33 GTS-T which is bond-released and fully federally legal. They're grandfathered in cars, which can't be seized.
As for the company, it's through International Vehicle Importers, which does everything legally.