Why do banks allow subject to financing?

[Doug Lovett]

I am under contract to buy a house subject to the existing mortgage.  This is my first subject to deal.  I'm wondering about the due on sale clause.  I've heard from several investors that banks virtually never exercise this clause and demand full payment.  My question is why not?  If the original borrower that they underwrote sells the house, why don't they demand payoff?