CAFE violations results in fines that are in the 10's of million dollars range per year, and higher when some of the manufacturers had to restate their cars' fuel consumption. BMW, Hyundai, Mazda, etc, all had to restate some of their cars within the last 10 years. The manufacturers' efforts to reduce consumption (therefore reduce fines) gave us, among other things:
1) the low tension piston rings that caused severe oil consumption problems in many cars.
2) problematic cylinder management systems (Honda 3.5L V6, GM V8, etc).
3) the very low rpm shift programming of automatic transmissions.
4) widespread direct injection deployment that isn't perfected for long term reliability.
5) 0w-20 oil requirements in the US is only a small part of it.
The car makers have to specify the exact operating condition used in the fuel consumption claims to their customers, therefore 0W20 is specified in the US. Outside the US, car makers are free to specify grades that provides better benefits at the expense of a slight (imperceptible to me) penalty in fuel consumption. 0W20 is fine for most of the US drivers, who never really stress their cars much. Those who wants a little more safety margin, take their cars to the track, and/or knowledgeable enough can select their own oil grade. The manufacturers' recommendation other than 0w20 outside the US is just another confirmation point that it's OK to use something else.
So everyone can be happy doing what they want to do with oil.