Why is Obama referred to as black when he is half white half black?

Because of the "one-drop" law that is a longstanding U.S. tradition, going back to the slave days. When you look at Barack Obama, you see an African-American. Consider this: Eric Holder, former Atty Genl, has two [b] black [/b] (light-complexioned) parents, as did Colin Powell. If you saw the three standing side by side, would you know who was actually "biracial" ? Obama also self-identifies as a black person, and he has every right to do so.