My guess is ISIS has a public reputation they want to uphold which is an amplification of what they're actually doing. By planting the story they benefit from t

My guess is ISIS has a public reputation they want to uphold which is an amplification of what they're actually doing. By planting the story they benefit from the extremist world view without having to actually commit the act that the story is about. Its also faster to make up a story and leak it than going out and actually finding corruption and do the act for real.