Some do, but not all. Scarfo would have any of his competition killed then inform on them to LE. Even some won't be forth coming with information if they are an informant, just bits and pieces to keep LE interested, and to make sure they are not in hot water with their handlers.

"I have this Nightmare. I'm on 5th avenue watching the St. Patrick's Day parade and I have a coronary and nine thousand cops march happily over my body." Chief Sidney Green