Here, let me try to explain in simpler terms...
The thing is, the whole "mortgage" concept add a lot of economic complexity, not to mention the emotional charge associated with the word, so let's forget for a while about it.
Let's suppose GB owns a house. Let's suppose I like GB's house very much, and would like to buy it from him.
I take a good look at the house, and find it's worth, let's say, $200000. In fact, even the market seems to back me up, as many similar houses have been sold for similar prices.
I offer GB 180000 for his house. He, noticing his neighbourhood is becoming infested with leftists like me, doesn't think twice and takes the deal.
A month latter, the prices of the houses plummet and I can't find even $100000 for GB's house.
Now, was GB being a thief for accepting $180000 for a house that's not even worth $100000 now? Was I being a selfish jerk for offering $180000 for a house I though worth over $200000? Are leftists really moving next to GB?
Not one thing, nor another, it's just the way the market works. Now, replace the sale operation for a mortgage operation. Remember, a mortgage is a lend secured by a property, where you either repay the borrowed amount with interests OR transfer the property to the owner. Usually, the value of the property is higher than the debt, but every once and then, the lender will make an evaluation mistake. Is that the burrower's fault? Of course not! Is he allowed to walk away from his debt by transferring his property to the lender? Of course he is!