They are indeed illegal in the US due to the fact that they are not an error proof system of mounting. The DOT requires that a tire and wheel combination cannot come apart.
They are also almost impossible to balance. Most people who use them don't even bother attempting to balance the tire, since they never get above 5 mph.
Using them on the street, especially on a very "un-rock crawling" type SUV like the Xterra would be pointless and well over to the right on the poseur scale.