i do not see this from most of the US left i encounter, or in the writings of the intellectual leaders of the left. chomsky, for instance, points out that he reserves his criticism mostly to US action (though he has written for example of british sabotaging of colonial india's industries) because (a) it is what he knows the most about and experiences directly and (b) it is what he can influence or change.
howard zinn has similarly limited his historical study mostly to america, and while eschewing the creation of mythical figures, he has highlighted the heroic actions ("goodness") of ordinary individuals.
others have mentioned that while the US may not be the best or the worst, it is the most powerful, and the effects of its negative actions are more pervasive.
otoh, people, like arundhati roy, IMHO the latest star in global pop leftism, do fly by NYC every few months with stinging, if shallow, critiques of the US.
i guess the analysis also depends on how we define the "US left". does it include [what seems to be] the million or so members of moveon.org? or is it restricted to strict marxists (how many of those are there)? or the few hundred cheering roy fans at riverside church? new age or hippie groups/individuals with misinformed ideas of eastern philosophy/religion?
--ravi