What’s the difference between preying on children as a criminal and preying on children as a professional?

I skimmed through a Rolling Stone magazine recently, my aging eye caught the cover photo of a shirtless Justin Bieber. I’m old and lame but not dead. Yet.
But I do read the words in addition to looking at the photos and right on the cover of the March 13, 2014 Rolling Stone, Issue 1204 below the JB headlines was this: The Cruelest Sting How Undercover Narcs Set Up an Autistic Teen. Inside the all too true story was titled The entrapment of Jesse Snodgrass. Here’s a link the the RS story and if you use the title as a search term you’ll find more thoughts on the matter.
Now I was hooked by one sentence, “He (Jesse) had resigned himself to maintaining a dignified silence when . . .” Oh how that struck home with me. I’m one of the lucky people. I’m intelligent and I had a good friend that no questioned asked helped me learn the social graces as a set of rules and then when Asperger’s hit the front page of Time magazine we knew why I was so different from other people. So I can relate to Jesse. What I can’t understand in my orderly rational existence is this:
If a civilian preys on a child sexually it is a crime, and rightly so because harm is done to the child. But if an undercover police officer, attending high school in the guise of a teenage student pressures legitimate highschool students into making bad decisions regarding drug use, the undercover police officer’s behaviour is legal and correct?
The heinousness of Deputy Daniel Zipperstein’s actions with e-mails and telephone calls and classroom conversations that ultimately stressed Jesse Snodgrass to the point Mr. Snodgrass regressed in his behaviour and inflicted damage to himself, all in the sake of obtaining a conviction for drug dealing, is perfectly correct in the eyes of our society and the law. But why?