Why single out race? There are other demographic factors that could count too: sex, age, social class ... and if a policy of "OK, conviction standards for blacks are lower" would be political suicide, a policy of "OK, conviction standards for working class people" would be even worse.

Even so such policies may indeed increase the accuracy of convictions, 1) they don't match our intuitions about justice, which I suspect would make people less happy, 2) judges and juries already take such factors into account (implicitly and approximately), so there's a risk of overcorrecting, and 3) energy would be much better spent increasing the accuracy of conviction with less ambiguous things like cameras, DNA tests, etc.

I do believe that such demographic data can be useful to help direct resources for crime prevention, though.