Why do "some" white people think every black person in America voted for Obama because hes half black?

Technically he calls himself black, but his political stage is biracial. Anyway, I didn't vote for Obama, as a lot of black people didn't. Most black people voted for Hilary in the beginning, and when she lost, it was either go with McCain or Obama, picking the worse of two evils. The candidate I wanted to win didn't even make it that far in the race. Most black people had no choice but to vote for Obama because hes democrat, and most black people are democrat, it had nothing to do with race. White people made up at least 80% of the vote that got him elected, thats mainly because of black people's 14% population density in the US.