FYI, they can be shipped to Commiefornia, it's just illegal to use. Although enforcement sort of depends on the region. For example, in my last car (which was much louder than my current), I never had any problem in the Greater Los Angeles area. But once I took a road trip up to San Jose (Central Cali) and got pulled over for aftermarket exhaust. The officer said, "You know it's illegal to have an aftermarket exhaust in California?" and I said "What? I had no idea!" He took a look at my license and said, "Well, I'm gonna let you off with a warning because I don't know how they enforce it down there." =D
So anyways, considering the more liberal interpretation of noise pollution in SoCal, I don't think my current exhaust will be an issue. Accelerating normally produces as much noise as an older 350z with slightly worn stock mufflers. And there's plenty of louder things on the road...like choppers (bikes), construction (jack hammers), traffic (honking), and exotics (occasional V12 supercar).
2011 Mustang V6 MCA PP Manual