This is a glitch. Players with the permission level of "custom" can use the game mode commands if cheats are enabled. They do not need operator status as required. Furthermore, if they change to creative mode, they can break blocks which respawn, fly and take fall damage, and see the creative inventory but not take items from it. This only affects non host players with the world having cheats enabled. This is game breaking for custom maps including my own.
A tip for map makers experiencing this: change all players to "Member" status.