Also, diesel trucks that emit black smoke do fail emissions test - in counties that actually have emissions tests.
You're talking about something affecting maybe tens of thousands of trucks, versus 11+ million vehicles including Porsche. It doesn't make sense to make the comparison.
The scale of an impact of one really bad car when it comes to certain emissions shouldn't be understated; it is important to remember that VW was simply seeking pollution they could get away with. Emissions tests are done on individual cars rather than a bulk standard by make/model for that exact reason.
VW is the second largest auto-maker in the world...few comparisons are going to hold up if that's how you think.
The EPA's goal is to regulate the overall quality of the atmosphere. To the extent that they regulate individual machines, it's just because of their contribution to the total.
When something is done by a few idiot hobbyists, it's just not worth trying to enforce. The effort would be better used elsewhere.
Individual cars are tested for emissions not so much to catch illegal modifications (although I'm sure they're happy to do so when they can) but to detect failures due to age or broken equipment.
"Rolling coal" is obnoxious and ought to stop, but it shouldn't be surprising nor upsetting that there isn't much enforcement effort against it.
It is also worth noting that the hobbyist solution has the potential of becoming widespread. If VW merely does a software fix, more than a few owners with suddenly sluggish cars might look into the devices. There is nothing specific about 'rolling coal' by trucks...most uses of the devices are purely for performance tuning.