Crimes against humanity and Torture doesn't apply to US but others

Last night I saw a very chilling yet true program about one of the Khmer Rouge's most infamous "enhanced interrogation" (sic!) prisons "S-21"
where among the numerous brutal torture methods used, water boarding was one of them.

The UN War Crimes tribunal is currently prosecuting the case.

Unfortunately, the US had a big hand in bringing Pol Pot to power.

There is a scholarly work on the subject.

Here's a cut and paste from the above article or link lest it be taken off.

The Khmer Rouge's most notorious prison chief told a Cambodia war crimes court today US policies in the 1970s contributed to the rise of Pol Pot's genocidal regime.

Kaing Guek Eav, or 'Duch,' the brutal director of the infamous torture centre S-21 said he believed the Khmer Rouge regime would have died out had the US not supported the right wing military government that removed Prince Norodom Sihanouk from power in a 1970 coup.

Duch, who is on trial charged with crimes against humanity and war crimes, made the claims as part of a detailed testimony of his own journey from maths teacher to fanatical communist revolutionary.

How come is it we call it torture (as it rightly should be called and prosecuted) but not in the case when the US government pursues the very same methods?

When I landed in Sydney more than 2.5 years ago, we took a cab from Sydney CBD to Chatswood the suburb where we used to live. The cab driver was a Cambodian and I apoligized to him as an American even though I wasn't an American at the time the atrocities where being conducted, thanks to our government.