Well, actually, the root of the problem has more to do with different properties that U.S. and European crash and emissions standards have.
Ahem...
In Europe, or at least large portions of Europe, there is abundant public transportation, plus you it's easy to walk/bike/take the bus to get around. Roads are designed for horse and buggy (ever tried getting around inner London?), gasoline is heavily taxed.
In the U.S., public transportation is more or less nonexistent outside New York City and a select few metropolitan areas. Therefore, a car is essential to get around. Roads are wide and gas is cheap.
What's more, California has unique topography that exacerbates smog caused by vehicle emissions. Cities like LA are nestled in valleys and surrounded by mountains, which can trap NOx and particulate emissions and lead to things like brown sunsets. Europe doesn't have these problems.
For these reasons, U.S. crash and emissions tests tend to use more realistic methodologies European tests. In particular, the Euro 6 standard is still based on the old NEDC test which hasn't been updated in essence since the 1970s and lacks real-world driving scenarios. The EPA Federal Test is based on realistic scenarios, and stricter on NOx and particulates, while EU testing only cares about displacement and CO2 emissions. This is why the Camaro SS, is heavily taxed in Europe even though it gets 27 mpg highway (the tax comes from its 6.2L displacement).
As for crash testing, EU/UNECE standards focus more on low-speed collisions with pedestrians, whereas the NHTSA in the U.S. focuses on high-speed collisions with other cars, again reflecting the different uses
It is for these reasons that Americunts can't buy a Hilux Diesel or an A45 AMG. And California's standards are adopted by the rest of the country, as well as Canada, because it's politically easy and California is a huge car market.