This kind of action is essentially an admission that the allegation is true. I'm sure there is a handy politician who could pipe in here and say "If you don't have anything to hide, then you have nothing to worry about..." Besides, it's the UN :)
If another country blocked a UN human rights investigation you know what would happen. The country would be instantly labeled as cruel dictatorship by the media.
it doesn't really make critizing foreign countries over human rights abuses easier for the US. and it's hard to argue against accusations of the US having double standards by governments like China's with this kind of news.