When we play chess with computer, why do we restrict ourselves to legal moves? We can make illegal moves but that means we are not playing chess anymore.

-- Dennett D. Kinds of Minds.

Social games are also like this: Why are we legal? We are playing the game because we want to win the game and we consider the game valuable. If the game ceases to be valuable, nobody will make legal moves.

All social games require a belief in the value of game. All ethics is based on such value. If a person doesn't believe that a certain game is valuable, they won't consider to restrict their acts by the rules.