The emissions inspections things were important during the late 70s-80s when they were really badly done on the American cars (you could disconnect the "smog pump" and get a better running engine). This is where the diesel exemption came from - trucks had nothing much at all (and I don't think they started adding cats for quite awhile, as dirty diesel (non-low-sulfur) was very common and would kill a cat faster than curiosity).

Nowadays the real emission inspection is the computer in the car itself, continually monitoring and tracking what is going on.