WASHINGTON (Reuters) - The head of a U.S. presidential panel on cultural property has resigned in protest at the failure of U.S. forces to prevent the wholesale looting of priceless treasures from Baghdad's antiquities museum. "It didn't have to happen," Martin Sullivan said of the objects that were destroyed or stolen from the Iraqi National Museum in a wave of looting that erupted as U.S.-led forces ended President Saddam Hussein's rule last week. Sullivan, who chaired the President's Advisory Committee on Cultural Property for eight years, said he wrote a letter of resignation to the White House this week in...