It's a carrousel scam. They are using funds from new users to pay old ones and at some point this model will -like all carrousel scams- no longer sustain, so the whole operation folds/pops.
Ehm....what? This doesn't make sense.
It's an exchange.
Person A has $100, person B has 1 Btc. They trade, Gox gains 1.2% for enabling this trade.
If you see the trade volume, they made tons of money doing this.
Unless they were screwing with the system by using the money/btc in there as in fractional reserve banking.