re: Why when a person is mixed (50/50 black/white), they are black and not white?

because being able to claim 'black' gives you a shite-ton of 'special' consideration - from excusing your sloth, to tolerating your incompetent, to giving you advancements you are not qualified for, to receiving praise for any action that was not catastrophic, to getting soft treatment if you are caught committing a crime, to shifting blame onto others for any mistake you make, all the way up to having nobody give a shite whether you live or die.