Honestly, I'm not sure if it's a case of not knowing they're racist, or simply having the tact not to spout it everywhere they go. Like the people who whisper every time they have to say "black people", not because it's wrong, but because they associate it with internal thoughts that make it wrong.
Then some, like my future mother-in-law, come at it from an angle of morality. As a member of the mainstream population, she's had it drilled into her head that racism is "wrong". So what's a person to do when they're both racist, and holier-than-thou? They call their racism something else, because surely someone so godly could never feel an emotion as base as hatred for another race.