This Saturday, government price controls on debit card interchange fees (which card issuers charge to merchants) go into effect. The controls are the result of the Durbin amendment to last year's Dodd-Frank financial reform legislation. They were enacted at the behest of big-box retailers such as Wal-Mart and Walgreen's, which stand to gain a multimillion-dollar windfall. But the controls are already transforming the retail banking landscape. The Durbin amendment tasked the Federal Reserve with establishing the allowable maximum interchange fees. It originally intended to slash them by 70%-80%. In response to a firestorm of criticism, the Fed cut the fees...