Come at me, bro

j/k
But you are indeed correct. If a motor wasn't sold in the states, it's considered grey market (illegal). It goes a bit more in depth, but it's the same basic principle why a RWD SR is still illegal as well. Nissan never offered that specific motor to the states in a RWD platform, so between the NHTSB and EPA, the motors aren't technically supposed to be in a street driven vehicle.
Not sure if there's a 25 year grandfather clause for motors the same way there is for cars, but given that there's no easy way to prove a motor's age, it would be a tough sell any way you slice it.