They are essentially writing a call option with no option premium. Nobody in the investment community does that because it is a money losing proposition. Why do you think there are no similar services for Gold or Silver? Except for an outright scam, a company would be out of its mind to guarantee cash refunds on purchase of a highly speculative investment.
Don't forget - as I understand it the basis of the whole idea is to build interest in DGC itself, by providing risk free entry into holding / trading DGC. This would surely benefit DGC as a whole on a far greater scale than just what the bank makes in profit. So if it works and people invest heavily in DGC because it is a risk free investment it would be mission accomplished with the extra bonus of a small percentage of fees as profit.