There actually was a two tiered system for a brief time in the early 2000s. California had recently passed stricter emissions standards, so all of a sudden the big three had a choice: make different cars for CA or make all their cars cleaner. They chose to temporarily make different cars for sale in CA while suing them in court under the Commerce Clause (because option three, don't sell cars in CA, was a nonstarter due to the gigantic market there). Meanwhile, a bunch of other states signed onto CA's standards, and the problem 'fixed' itself. It was an interesting but very temporal situation in the industry. Everyone knew it was temporary and that's the only way it occurred in the first place. Not that this is very relevant to the discussion at hand; just wanted to share a bit of history that I doubt many people are familiar with.
They had a new regime, the Zero Emission Vehicle program, that took effect in 2003, and applied to all new cars starting in MY 05 that created a huge legal mess in which the EPA and the auto industry jointly sued the state of CA over their standards. In the end, CA prevailed, if I'm not mistaken. (I only know this because I interviewed for a job as an emissions engineer for DaimlerChrysler in late 2005--didn't get it, but still had to study up a bit.) But you're correct that it had nothing to do with safety of the car.