Well I was inspired to do some research and it appears that the gov assumes all imported vehicles originally manufactured for on road use are on road vehicles, regardless of their intended use. It appears too that it must then pass safety and emissions in order to be imported. So in this case, a vehicle that was built for on road use was brought in without being DOT and EPA compliant, which makes it an illegal importation.

I was under the impression as long as you were not driving on public roads you would be good to go.

i love a boy in uniform