They just hate the U.S. in Iraq. Shiites and Sunnis are as one against the American occupiers. Muqtada al-Sadr is regarded as a national treasure by his co-religionists. All Iraqis welcomed the deal the U.S. brokered in Falluja, which averted the demolition of the city. Shiites oppose an American entry into the holy cities of Najaf and Karbala.
Any more dead certainties you want to throw our way? If so, you might want to skim this New York Times article by John F. Burns.