This is a very common Fantasy-genre trope. It's a way of explaining away various others, such as Joker Immunity, Failure Is the Only Option, etc. Basically, Good and Evil are unionized, and have rules they must follow. These rules are usually towards overall self-preservation; no one side is allowed to "win" too much, and the Big Good and Big Bad restrict their fighting to a Divine Chessboard rather than going at it in person. Examples of Balance Between Good and Evil include: