I have no statistics on it, and should have prefaced the statement with, "It's my experience that..." Whether or not a lender will "gift" the deficiency should depend on the owners' assets and ability to pay. Remember, if the lender does "gift" the deficiency, the IRS may come around with it's hand out, saying that the "gifted amount" which you "received" is taxable! There are cases where people will be able to show that they cannot pay the IRS tax, but I guarantee you that the IRS will be staffing extra ppl to work on trying to collect it anyway.
One last thing that I would like to add is that the phrase, "the seller walks," bothers me a bit. It almost sounds as though there is zero consequences to the seller, but that is far from the truth. They may be able to get some debt problems when they "walk away," but credit is hit, and for someone in financial trouble, that may be just the start of the financial problems forthcoming. Sure, the real start came earlier, but this is when others will begin to squeeze harder.