The vehicle were inspected by the US but due to the "cheat" it passed their regulations. However, as soon as the vehicles are driven in real world conditions, they no longer meet the emission regulations, and as such are not suitable for use on US roads.

The vehicle you cheated on the emission test many years ago, is a case in point. The vehicle only passed the emission test because you modified it. As soon as the vehicle was back on the road, it was breaking emission regulations again. That may have pleased the vehicle owner by saving him money, but could have cost him more had he been stopped by the "local council" with their mobile emission vans (which never materialized).

Currently, the VW owners in the UK pay less roadtax based on their emission output. If the figures are incorrect in real world driving, then they are cheating the government out of thousands of pounds of revenue, and governments definitely don't like that!