MtGox most definately CAN print money. The number they show you as your balance at MtGox can be purely printed money which they cannot back up with anything. And that means both USD ans BTC. MtGox can print both USD and BTC. The difference to most banks is that banks are audited and that limits what they can get away with. Nobody audits MtGox so they can print all the money they want all day long and it only causes problems when too many people want to withdraw.