Secondly, only an international body could have the neutrality to investigate human rights violations in all nations. Or would you prefer America to continually submit to human rights investigations from, say France? I know the French or the British wouldn't stand for the Americans investigating their internal affairs.
Why? Because all nations are guilty of some human rights violations. None of the nations on earth today have a perfect record. So for America to investigate France, or for France to investigate America would be rather hypocritical.
The UN, however, while not perfect in cataloguing every human rights issue on Earth, at least has a neutral eye towards it.
The idea that the UN is somehow specifically anti-American is ridiculous, and moreover, arrogant. The UN is made up of all the member states of that organisation -- America is part of the UN, and thus, a part of the UN IS America. It can't be anti-American. It can simply be fair, which can seem very unfair from nationalist standpoints.