Sieges Question

Can anyone explain the rationale whereby as the defender playing black, a draw LOSES the board ? Surely the imperative should be on the attacker to win ?

Originally posted by cambridgeian
Can anyone explain the rationale whereby as the defender playing black, a draw LOSES the board ? Surely the imperative should be on the attacker to win ?

I guess it's something to do with making it harder to keep. It means that the defender has to attack, rather than simply playing for a draw. Thus allowing them to cycle, rather than one good player keeping it for all eternity.