Because once upon a time there was (and still is) a thing called white supremacy. They believed they were the standard. So the one drop rule was created. Essentially saying as long as u have 1 drop of X race u are now that race -- like u were tainted or something. Also u may be mixed but essentially u are what the world (and police) see u as. In other words if white is the absence of color, when u add "color" to it it never looks the same.  So while Obama might be equal parts white and black ... His experiences were probably overwelmingly black. Ergo ... He is "all" black by societal but not geneological standards.