So, Why Did...
... the Americans stop short of calling this a defeat device? Is it because it's on American designed and manufactured trucks?
After all, the vehicles have passed whatever the testing regime threw at them, just like the VWs, but now there's a question mark. Something is clearly wrong with how the vehicles behave outside the test lab, just like the VWs.
In VWs they called it a defeat device, but in Chryslers they've not done that, yet.
Could it be that the relevant authority knows Trump and his climate-change denying chums are days away from being their boss?