Why are all black Republicans all radical right wing birther Tea Party nuts? Christ, can't they wheel out a moderate token black Republican ?
My guess is that since most Republicans are uncomfortable and downright hostile to blacks at times, is that they need to show that they are true believers.
My assumption is that moderate blacks generally have no reason to cross over and become Republican. Only the far-right wackos are motivated to actually change parties, but your guess probably has some truth to it too.
