Instantaneous exchange/conversion of bitcoin into cash
Because the exchanges are in possession of confirmed bitcoins from the get go. Mtgox makes you wait about a full hour before you are accredited deposited bitcoins to get those 6 confirms. And to exchange it they just decrement the person's account's bitcoin balance and increment the fiat balance (according to whichever market order).

On coinbase they just decrement your account balance and send fiat to your bank account simultaneously.