In a Gamist game like D&D, GMs certainly can 'cheat' IMO - eg secretly altering a die roll to enforce your own preferences against the wishes of the players; eg to keep a pet NPC villain alive - definitely _is_ GM cheating in my book. Altering a roll to favour player preferences is ok if the social contract at your table allows for that; ie if the players are ok with it. Likewise players cheat when they deceive the GM & other players to favour their own agenda; by altering a die roll, giving themselves extra hit points, etc. In a typical Gamist D&D game, this is NOT OK. 
