Go ahead and fight it in California if you wish. Two things to consider though.
First the exemption is for the crash test of the bumper not whether a plate can be attached. The ONLY thing most judges would care about is that the plate is attched, right there.
Second, and probably more important for these kinds of laws, is that for every ticket that is admittied or found guilty the court will order you to pay an ADDITIONAL 50% of the fine to the "Courthouse Construction Fund". That's just how they build them around here.
Judge sure wants a new courthouse and after an unbiased trial they let you know how much you shall contribute. Sorry, the law states NO tax money to be used. They gotta earn new offices. I'm sure new offices will give better decisions in the future.