Hume's Law: you cannot derive 'ought' from 'is'; in other words, factual premises cannot entail an evaluative conclusion.
The supervenience constraint: the moral facts are, in a sense, 'fixed' by the natural facts. There is no possible world that is exactly like our own one in all factual respects, but where it was 'morally right' of Hitler to carry out the Holocaust.
It seems to me that there is some tension between these two principles. If the natural facts 'fix' the moral facts, then wouldn't that set up an entailment relation between them? If you had an accurate theoretical account of just how this 'fixing' was determined, couldn't you reason straight from the natural facts to a moral conclusion? (And don't we attempt to do this all the time - albeit with imperfect natural and theoretical knowledge?)
So why does anyone believe Hume's law?