Those who default and walk away from a property are essentially saying, "we are failing to live up to the contract we signed, and per the agreement, are willing to forfeit the property. It is not our property."
But do the mortgage docs make for allowances of "failing to live up to the contract"? I've never read one that says here's your 30 year installment plan unless you lose your job in which case you move out and we'll just call it a day. They're still on the hook as far as the contract is concerned, maybe state laws mitigate that responsibility but that's a separate concern. From a contractual stand point reneging and walking or reneging and staying seems to be comparing shades of grey.