While I think that everybody was fine with nazi Zombies, I think vietcong zombies would piss to many wannbe hippies off. And they would have a point, all the veitcong did was to dare disagree that America is always right all of the time. (Yes I know they commited war crimes agaisnt thier own people, but china has done that on an almost daily basis for most of the last century, as have the serbs and bosnians)
Though I do hope we get a zombie mode, I am not the type to get all weepy about shooting pixels