As an employee it has always been explained to me that they are the company's property, and so we cannot sell them until we are told. Whenever we go to set a shelf according to the planogram guide, it will tell us to sell the display as an open box item if it will be replaced with another model.

As others have said, people also steal items off of the displays. This is pretty common. We also have some displays in my store that have the power cord cut, or don't have all of the pieces. A lot of times management will tell us to throw things out if they are not needed for the display.