Like homeowners walking away from mortgaged houses that plummeted in value, some of the largest commercial-property owners are defaulting on debts and surrendering buildings worth less than their loans. Companies such as Macerich Co., Vornado Realty Trust and Simon Property Group Inc. have recently stopped making mortgage payments to put pressure on lenders to restructure debts. In many cases they have walked away, sending keys to properties whose values had fallen far below the mortgage amounts, a process known as "jingle mail." These companies all have piles of cash to make the payments. They are simply opting to default because...