Diesel Cars

Will someone please tell me why diesel automobiles, which are major polluters, much worse than gas engines, are exempt from the pollution requirements? I was surprised to learn that diesels are exempt from smog checks.

Am I being too cynical in suspecting that something is very fishy here? This engine, especially in passenger cars, should be banned, not exempted. Has Germany (Mercedes) found a way to get even by choking us to death?

DONALD B. BROWN