How come just because Obama's father is black that means he is black?

it makes me so mad when mixed people are half black and when they do something great they acknowledge the fact that the person is black...but obamas mother was white...so why isnt he refferred to being white?......as far as i see it he is half and half...and tiger woods isnt one hundred percent black..but since he is such a great golfer the media credits him for being black