It's very easy to explain. there is plain old vanilla criticism, which is fine, then there is criticism which comes with a thinly veiled (and in some cases, not so thinly veiled) implication that Americans are some retarded sub-species of humanity and that the world would be better off if we all just commited mass suicide (if you want an example, I can't find the thread but I seem to recall someone a year or so ago making a post entitled 'The solution to the worlds problems' which consisted of a map of the world sans the US. A good many European posters thought this was hilarious).
Not all criticism of the US is of the latter type but it is common enough that one hears a lot of it, and it is often preceded by the first type. Once you hear enough of the latter type, you begin to get reeeeaaaalllllly suspicious that anytime you hear the first type of criticism that the second isn't far behind. Thus defensiveness.