When a primary residence is sold for more than it was purchased, the gain is generally subject to a large Capital Gain tax. If the home is granted a short sale, the difference between the sale and mortgage value is also taxable as income. However, if the residence is sold for less than purchase price, the loss is not tax deductible.
It is predatory for the United States to charge a Capital Gain tax on any housing that is sold for more than purchased, but not allow a Capital Loss deduction when the reverse occurs.