Did U.S. troops encourage the looting?

This site published an allegation that the occupying troops declared open season on looting. Was this a propaganda effort (look! freedom!) that got out of hand? When you couple it with reports that much of the museum looting was “professional” in nature, it looks pretty ugly. (Not as ugly as maimed human beings, but centuries from now it’s the cultural patrimony that may have disappeared already into private collections that will be missed.)