The vast majority of offroad equipment is not farm equipment but operates in urban environments. As NOx is an air pollution concern, there should be different regimes for rural areas versus urban areas. Construction equipment operating in urban areas is different from a tractor on a farm.

> but US tractors fall into the much less restrictive off-road category.

Sometimes. Above 26HP tractors do have to have emissions controls like diesel particulate filters now. Below that they don't.