Because an attack taken at no penalty is a strike to kill through whatever means happen to presented in combat. One's HP is what prevents one from suffering crippling or fatal injury, until those are depleted that foe has the right to continue to fight at full efficiency. If you want to take their hand rather than kill them outright, you have to limit your strikes and you still have to deplete their HP. I chose -4 because it is the same as striking for non lethal damage. Also that called shot bypasses -10 and thus taking the limb of someone in diehard means no last ditch rage/ frenzy with a 2 handed weapon.