Maybe they can just make different models with different capabilities and those can be sold in accordance with the standards of the markets in which they are sold.
In most cases, it's FAR cheaper to make one single model that complies with all of the market standards - that is, with the highest standard found in any market.
In the case of the US, many cars are sold that
exceed the crash safety and/or fuel economy requirements even of California, because they already meet higher standards imposed by the EU; While in the case of the EU, many cars exceed local NO
x standards, because they comply with the stricter Californian rules.
So the Californian standard, or any US Federal standard, is utterly without effect for cars sold in both the US and the EU, whenever the EU standard is highest. And the EU standard has no effect when the Californian standard is higher.
US automakers who build models not sold in the EU need meet only the standards set by the strictest US state (usually California), but if they plan to sell to the EU, there's no benefit in making two models that differ only in their adherence to the different regulations, unless by doing so they install a completely different engine - selling the same model with a few different engine and/or transmission options is already a commonplace.
Basically any sufficiently large market region can dictate higher standards to automakers, that will flow through to the models sold in places with lower standards. For this not to happen, the low standards market would need to be massively larger than the high standards market.
So you get crappy cars sold to India or China, but not to Australia or Alabama, because there are too few Australians or Alabamans to make it worthwhile to build a special middling-standard model just for them.
If your local standards are set too high for you to buy Indian or Chinese clunkers, then by default you will get cars that meet EU and Californian specifications, and exceed any local standards.
If it were cheaper to build middling-standard compliant cars, then they would. But it's not, so they don't.
TL;DR - Economies of scale impose a defacto single national (or even multinational) set of standards that is equal to the highest standard set in any sub-region. It's of zero relevance whether the standard is set at the state level or at the federal level, it will be complied with (or exceeded) nationwide anyway.
