Why do the traffic police lie in wait?

Why do the traffic police lie in wait? Why do they wait till you jump the light and then pounce on you?

I haven't understood the logic of this, ever. I am not an legal eagle, but I do know that it is job of the police to prevent crimes from happening. How is it that in the case of a traffic offense they wait for it to occur and then pounce on the perpetrator?

I mean, how come they lurk behind the lights at the intersections and catch the guys jumping the lights after the latter have committed the offense. Why don't they stand in front of the traffic lights and prevent the crime from happening in the first place? How is this offense different from any other offense?

Am I missing something?