Blacks supporting Obama?

I just heard Rush Limbaugh's commentary about the Powell endorsement (watched it actually) and he screamed at the top of his lungs that Powell's endorsement was "totally about race." Why is it that blacks are put in the position of justifying their support of Obama and then summarily dismissed as being racists who are only supporting him because he shares a skin color? This is has been going on this entire election but I have to say that these allegations against Powell piss me off more anything else in this election cycle. This man served his country for 40 years, stood on a stage and spoke very eloquently about how Bush and Chaney were going to heal the racial divisions in the country and put his a$$ on the line for the republican party regarding the Iraq war when anyone could see that he didn't agree with what they were doing. If this is the mentality, why shouldn't anyone ume that those who are supporting McCain are only doing so because he is white? Same logic right? For that matter, why are whites who support Obama being portrayed as simple minded individuals riddled with white guilt who only want to prove how non racist they are?