I think he's just saying that not all rational evidence should be legal evidence. I don't think that he should be read according to LW conventions when he calls lower evidence standards for blacks a "rational policy". He doesn't mean to say that it would be rational to institute this policy (and yet somehow also morally abominable). He means that institutionalizing Bayesian epistemology in this way would be morally abominable (and hence not rational, as folks around here use the term).