Derek Parfit talks about the fact that you are allergic to walnuts is a good reason not to eat them. Thus, and with other examples, Parfit tries to connect facts with reasons/desires and hence off to providing a normative ethics. BUT, being allergic to walnuts is a reason not to eat them only if I'm not suicidal or in love with the pain of an allergic reaction. It certainly provides no reason why I shouldn't encourage an enemy to eat them if they're allergic -- or to do it to satisfy my own sadism.
Facts and values are intertwined, I'll admit that, but there is NO reason to think they are uniquely paired or united. That's the whole point of relativism/nihilism etc... Fact-values, or fact/values, can be organized in many nonisomorphic ways. Parfit's arguments are useless. UNLESS, gentle reader, I'm missing something crucial. See, my neck is exposed. I DARE you to respond. BOK! BOK!BOK!