They are voting for an effect, a consequence. This effect is the diminishing of a power imbalance. The power imbalance is a wrong in our society, and they are voting to correct the wrong.
A white person voting for a white person because he is white is voting to uphold this wrong power imbalance. There is no reason to do this that is not malicious.
THAT SAID, I still think that it's not a very good idea to vote on race alone, no matter what race you are. I have great hope in Obama; I believe he will do great things for our nation--but if I'm just going by the color of his skin and not his platforms, I don't know that he's going to enact anything real to correct the power imbalance besides be a black man in office. But being a black man in the office of potus IS significant; it IS a start because of its symbolic value. But one should hope--expect--that he does something beyond exist as a symbol. I believe very much that he will.