U.S. Fails in Effort to Block Vote on U.N. Convention on Torture: ... Diplomats say, and American officials do not dispute, that the United States is sensitive about this issue because of potential demands for access to the detention camp at the United States naval base in Guantánamo Bay, Cuba, where more than 500 detainees suspected of being Al Qaeda members and others seized in Afghanistan are being held, as well as to others held in the United States as "enemy combatants."

Why on earth would THAT be an issue? The administration has allowed the Red Cross to visit. They've certified that they saw nothing untoward going on. Unless things have changed drastically since then, there's no particular reason they should worry about visits to Guantanamo.

But an American official here said that the concern in Washington was that the plan, couched in what is called an "optional protocol," would be unconstitutional in the United States because it does not recognize states' rights. Adherence to the protocol would be voluntary. He predicted many other countries would also refuse to cooperate because of its intrusiveness.

... How, precisely, is a voluntary protocol going to be unconstitutional? If a state doesn't want to allow inspection (and none of them will), they simply refuse. (There is the broader question of how a voluntary protocol is at all useful, but that wasn't the administration's concern.) Something that allows you to refuse without penalty can't be unconstitutional as such.

A U.S. official, speaking on condition of anonymity, said allowing outside observers into state prisons would infringe on states’ rights.
[...] The anti-torture proposal enjoys wide support among Western European and Latin American countries. But conservative Muslim states that shun outside intervention are likely to back the U.S. request in order to stave off a vote. Other opponents include Cuba, China and Nigeria, human rights activists said.

So basically, we're now on the side of Islamists and countries we consider intensely repressive or just don't like at all.

Well, all-righty, then!