In the 90's I worked on vehicle emissions inspections programs for several states. A colleague of mine wondered if what we were doing was cost effective. He found out that it was cheaper to buy all the high polluters that it was to run the inspection programs. Of course nothing came of it because it would have put us out of business.
If the government was buying high-polluters, though, there would be 10x more of them. People would keep them running and sell them to the government rather than junking them.