I don't know, but from time to time I see police strategically located by the entrance to a freeway on-ramp, staring into every car that passes by, with other police pulling ID'd cars over. Having driven through "the stare" it's pretty clear they're looking for people not wearing their seatbelts.
How do they prove that one? Take a pic?
Maybe they are hoping the people dont have time to fight them in court.