If you’re playing chess with another person and you decide to shove the bishop up your ass, you are effectively no longer playing chess. This is because the game of chess only exists as a social contract between the two players (given the both know the rules). In a game of computer chess this social contract needs to be written into the program’s logic. If the “computer” decides to make an illegal move against you, does that mean it is effectively no longer playing chess? After all the computer is violating the social contract of chess. But the computer is simply following the program’s logic precisely, the social contract is only violated by the programmer’s inability to correctly write it into the program logic. It is like playing chess against someone who does not know the rules of chess.