How about,
"Thou shalt not lie". I've bought several houses and no where on any mortgage agreement that I signed did it say "I'll pay the money back, as long as my house isn't upside down. They have all said, "I agree to pay $X amount back." You walk away when you can still pay the mortgage, the your fruits tell that you had lied about your intentions. Once again, there is a difference between not being able to, and not wanting to.
"Thou shalt not steal". People who can pay their mortgage, but walk away, because their investment has gone south have for all intents and purposes stolen the lender's money; leaving them holding a property that is worth less than what was agreed upon. One could also debate that since foreclosures seem to depress prices in any given area that they are stealing from their neighbors, too, as their neighbor's property losses value due to their actions. Once again, there is a difference between not being able to, and not wanting to.
If the contract, or state law, which is incorporated into the contract, permits one to walk away, it is not stealing. The lender surely was aware of this risk at the time the mortgage was given and adjusted its rates or closing fees to account for this.