The way this game is set, America just represented the white people. All the unique units and leaders in all Civilization series always shows just the white heritage in USA nation. Also, when I try to argue in this forum about the Native American heritage in USA, all user desagree with me, because they thought USA as a pure white society.
The civil war had black troops, it can be a Unique Unit. Black Panthers, Mahowak Rifle club, both can be units for USA. But not, that time is the Cowboys, last time the Minuteman, always just remember the white heritage of America. Also, all USA leaders was white, why Roosevelt who kill Cubans and Native Americans deserve to be a leader and Barack Obama not?