Well they get lots of fines and fees from court and jail when someone is charged. But even if the stolen plate isn't really stolen its an easy excuse to pull someone over and catch them for other crimes. It's why cops sit around fast clear roads and pulling everyone going 5 over but aren't giving speeding tickets. They are fishing for usually drug related crimes but also people on probation or behind on their license or tags, or technical DUIs if someone admits to taking a narcotic at lunch. They legally can't pull law biding citizens over randomly just to check, so they need and find excuses to do so and shitty ai flags are good enough.