And how, exactly, do you verify PINs if the network is down but you aren't locking out customers? There are many millions of ATM users on each of the major networks, and every ATM on the network needs to be able to authenticate every PIN. Which is worse, having to update a massive file of account numbers and pins, allowing users to withdraw without pins, or making users go use another ATM (or, God forbid, an actual live teller)? Yes, it's inconvenient to the user, but so is overdrawing your account, and so is having a thief overdraw your account.