Not wearing your seatbelt is a relatively minor infraction of the law. The person you are most likely to harm is yourself if you're in an accident. Don't these cops have actual crimes they should be taking care of?
And why do they need to pose as panhandlers if they really are concerned about this crime wave of seatbelt scoff-laws in Washington? A person can get themselves a lawn chair and just sit near a street light on the sidewalk and watch the passengers and drivers of the cars that go by. You can get a pretty good look at the people inside a stopped car at a street light. It would be much easier. Or stand out there holding a sign that says "Lost Cat!" or some other neutral thing. Sounds more like a crack down on panhandlers to me.