Isn't it the other way around? The police are asking permission to synthesis abuse images so they can pose as abusers in order to access forums of those actually perpetrating or encouraging abuse. The parallel given (I read it using Google Translate) is of posing as a drug dealer.
I'm sure some of them do. The last sentence makes a parallel to cops posing as buyers of illegal drugs, not purveyors, though I expect them to want that as well.
Seems reasonable to me?