Yeah, I've seen a depressingly large number of the first lot. (Though that number is still only in the low double digits, any is really a depressing number).

I think it is as simple as them being idealists. They've "read Pol Pot" or whatever and assume that because he wrote something halfway decent on paper, all the of the actual actions of the Khmer Rouge are irrelevant.

Alternatively they might take the attitude of "the west lies about every other AES, therefore everything bad that happened in Cambodia was also a lie."