Why is Jem considered a "traitor" in To Kill a Mockingbird?

Dill runs away from home one summer, and Jem and Scout find him hiding under the bed. Jem insists that they tell Atticus, but Scout feels Jem has broken a law of childhood by betraying Dill. This shows how Jem is maturing because he understands how worried Jem's parents and Aunt Rachel will be. Scout and Dill see Jem as ratting Dill out to the adults, taking their side over Dill.