I don't get this. Banks must always do this, they exist because everyone else wants to lend short term or borrow long term. Anyone who acts as the counterparty to that is automatically a bank or bank-like.
I guess what you mean is that when the banks borrowed short term (as they must), they did it using home loans as collateral, and that more of those loans defaulted than was expected.
How does that justify defaulting?
But that's not really what happened. Banks made a lot of poor quality loans, then packaged them up and sold them on, taking a spread in the process. Some banks (e.g. Bear Stearns and Lehman) kept some of that risk on their books, but others (Goldman) didn't.
This wasn't a case of banks using their mortgage book as collateral, it was banks originating and actively marketing a dangerous product. It's also worth noting that some banks knew that the products they were selling were low-quality and likely to default.
Then the US government stepped in and propped up the instigators of the crisis. People who made poor borrowing decisions had a part to play in the crisis too, but I think it's fair to argue that the pain could have been distributed more evenly.
Under the circumstances, I can see why it might seem fair to walk away from an obligation like the author's mortgage.
How can you expect people to not default in a system endemic with corruption and rife with irresponsibility?