This started out as a comment on Alex Tabarrok's post on Marginal Revolution, where he expressed shock about an article in Counterpunch headlined "Cambodian Genocide Denialism in Counterpunch."
I've not read the Counterpunch article, and I probably won't because I agree with Tabarrok's assessment — and anyway he's far smarter than I am. However, I did live in Cambodia for several years, know many Cambodian-Americans and like to think I can provide some context.
There is a strain of denialism in Cambodian society. About half the population was born after 1980 — so half the population has no direct knowledge of the events. A great many Cambodians who survived the experience don't like to talk about it. There's a cultural aspect: It's considered bad luck to talk about bad things.
And some people — young and old — simply don't want to accept that Cambodians would massacre other Cambodians. It doesn't help that the current regime, which is not considered a paragon of democracy and liberty, uses the genocide and the Vietnamese-backed invasion that ended it as a significant source of legitimacy. There is a sense of "if the dear leader says the sky is blue, maybe it isn't."
It's not uncommon to hear people — even educated, worldly Cambodians who have studied and lived abroad for decades — muse that the Vietnamese orchestrated and planned the genocide to erode Cambodian power. At least some former Khmer Rouge soldiers (especially the holdouts on the Thai border) have expressed no remorse at what happened because they say they were defending the homeland against foreign threats.
On the issue of genocide versus class-based killing, everything I've read and heard indicate that the killing originally targeted the "urban" class (a class broad enough to include anyone who had lived in a city, spoke a foreign language, or had even passing contact with a foreigner) and rapidly spread to anyone who could be considered a threat to the regime. Toward the end, the regime was demanding almost a quota of traitors, leading to random people being rounded up and killed (at least in some areas — enforcement seems to have been uneven). Of course, there are many scholarly works on this subject and reasonable people disagree on the details.
In short, it's not surprising that someone could travel to Cambodia and find a few deniers. I would like to think the U.N. tribunal is helping to change that, but maybe not.
Anyway, if you prefer a first-person account instead of watching an academic grind an ideological ax, check out this reddit question-and-answer session with a Khmer Rouge survivor and her daughter.
That's something worth reading.