Here is how MtGox works: They make a cut off every buy/sell transaction.  That's how every exchanges make money.

When you withdraw $, MtGox pays you from it's holding accounts.  That $ got there either by your deposits or the sale of your BTC.  MtGox doesn't pay anyone unless someone else ponies up the $ or the BTC.

When you withdraw BTC, they are yours so MtGox gives them back to you.  MtGox doesn't pay anything.

Now if MtGox operating expenses outpaces their revenue, then they might have to dip into the holding accounts, but that would be a very bad way of doing business.  I'm sure they make enough profit on every transaction to pay everything and then some.  If they didn't, they'd just increase their cut to make more $.