If I was operating this service, I would just sell the Bitcoins as they were deposited to me. When people wanted to withdraw, I would buy them back at the current market rate. Of course I would do this in bulk and have a small reserve pool of Bitcoins to reduce my fees and improve liquidity, but that's not very significant.

As long as I can buy/sell for cheaper than I am charging my customers (which I can if I'm using MtGox at 0.5% and charging 0.75%), this is profitable with no risk to either me or my customers. There is no need for this to be a scam or a gamble by the operator (although it doesn't exclude the possibility, obviously).