They were not. That's B.S. SP loans were just a means to an end.
Indeed, pick up any big-city newspaper circa the late 1990's or early 2000's and you'll see that most large cities were using every tool available to push banks into loaning to people who didn't have the financial means and history necessary. I remember when I moved from northern Ohio, Cleveland was in a legal struggle trying to force Third Federal Savings and Loan to lower their lending standards, accusing the outfit of "redlining" neighborhoods. ACORN was in on this debacle as well. Freddie and fannie provided cover to protect the institutions.
But the problem grew, and it wasn't just low-income borrowers riding the train of easy credit. Interest rates that were too low for too long compounded the situation - Greenspan dropped the prime rate after 9/11 but kept it down way too long, encouraging more leveraging (borrowing). There's many other factors involved, but suffice to say that the housing bubble rode on the credit bubble and when credit ran out, housing was bound to fall. Unfortunately, the fall has taken many responsible people with it.