I had guessed that you had more than a typical interest in the topic...
Fyi - I had heard (unconfirmed) that volkswagen had a similar high efficiency car that was not available in the USA for the same reason - slightly high CO2 emission in a very high MPG engine.
I agree with your overall point that there is some derivitive of deisel technology available in Europe that is very good in fuel economy. Plus it is not the choice of the car manufacturers, it IS directly related to our government’s regulations!
I even have a friend that used to live in Windsor and he was able to get a Jetta that was not available in the US. He loved that car for many reasons, but he bragged most about the diesel engine’s mileage!
;-)