It isn't "banned"
VW has made no efforts to make a version that meets our highway safety standards. US oil companies are not stopping them from doing this.
VW is only producing ~2000 of them. Why? Probably to limit supply so they can charge more for the same thing. Again, blame VW, not US oil companies or government.
Most of the uninformed Americans tend to think of something like a two stroke detroit when they think of diesel, and it's more expensive. It's not a selling point for "save the environment" or "save on fuel" advertising. VW knows this.
People here want gasoline-electric hybrids, or electric cars if they want more economy or reduced emissions. That's just what sells, likely due to advertising by companies other than oil companies.
Just because VW hasn't decided to sell a limited run expensive diesel here doesn't mean it's government/oil company conspiracy. Think about this stuff a little more, people.