Emotional response usually trumps intellectual understanding, for pretty much everyone really.
And I do not think we should be singling out the Americans, a continent with over 300 million people, and saying that they are uniquely prone to this kind of behaviour.
As for the KKK, at their height they had a very large membership, thats true, but the sensibilities and understandings of the time were very much different from today - thats not to excuse their worst excesses, nor their racial supremacist agenda, but to explain it.
Nowadays, there membership is barely into 4 figures, so not much better supported than the BNP over here, or Golden Dawn in Greece.
The appeal of such organisations rests with offering simple,direct and unequivocal solutions to complex multifactorial problems -and on the healthy fear and paranoia of many humans who might feel livelihoods or ways of life are under threat.