I wish the Islamopologists would pick another example of white, Christian, American terrorism to defend Islam with. Oh, wait. There really isn't one.
That was the problem you posed. The answer is, there are (more than a few) and I just named one of the most obvious and notorious. I wasn't making excuses for terrorists on the other end of the spectrum, nor was I implying that mainstream American Christians didn't stop the KKK. There is no need to be reactionary.