What happens when they sell "under water." Let's say they purchased for $400,000. They agree to pay 25% of their income for the mortgae. They sell for $300,000.00. They pay $75,000 to the bank, keep $225,000.00, but continue paying the 25% of their income to the bank.
This evens the moral playing field between those who do what might financially best for them, and those who feel a moral obligation to continue paying their mortgage.