Matt Finch addressed the question: Can a Dungeon Master cheat?
I don’t think an RPG is a game. Also, for RPGs fiction and rulings are more important than rules. So I’m not convinced that the concept of cheating applies at all.
I do know that a referee can be unfair. Although it isn’t a clear cut & dried thing. The rule I hope to live by is that it is better for a ref to spoil a surprise than to seem unfair.