If you do a chargeback the bank double checks this, investigates it and people who repeatedly try and do fraudulent chargebacks get their accounts terminated. It's not like your bank offers you a "reverse this payment" button in the UI that always works, right?

 

If you attempt fraud against a bank, they know who you are and will come after you in one way or another. But it's safe to assume that users of a double spend service would be anonymous and the kind of merchants they go after are not hassling their customers with strong ID checks, so there would be no consequences for them. It's a game they can only win.