Part of this one is straightforward. The US opposed the Khmer Rouge takeover in Cambodia and did publicize the regime's crimes. However, the Khmer Rouge proved to be virulently anti-Vietnamese. Border skirmishes were followed by a full-fledged invasion that ended the Cambodian Genocide and toppled the Khmer Rouge government.

Now comes the twist: Because Vietnam was backed by the Soviets (and perhaps because we had lost the Vietnam War), the United States wanted to act against the expansion of its influence. So we secretly aided the Khmer Rouge (who were more or less backed by China, our friend of convenience during the later Cold War), who we had been bombing a decade before. Our diplomacy naturally downplayed the crimes of the Khmer Rouge while we were aiding them.

One last note: the convention against genocide doesn't actually prohibit the annihilation of a group of people because of class, which is how the Cambodian genocide started. Under the Convention, the genocide didn't clearly begin until it targeted ethnic Vietnamese. Of course, this definition has been contested on the grounds that it was Soviet insistence that kept class out of the list of protected categories.