The situation only makes sense if you consider a separate entity called "American Christianity" which is an entirely separate religion to Christianity. Not a branch of Christianity, not a form of Christianity, but something with absolutely no connection to Christianity at all. It's a separate religion. And what is the goal of this religion?
look at it phenomenologically, look at it sociologically, and what do you see? Basically a syncretic folk religion, based primarily on American nationalism, an expression of the "pervasive religious dimension of American political life". (Bellah; see also "Civil Religion in America") Its purposes are basically civil and political. Its morality is taken from a highly selective and individualistic reading of the Old Testament, and it mixes in bits of consumerism, Zionism, Republican political values, and corporatism for good measure. Add to this an almost romantic sentimentality concerning the person of Jesus, much like the contribution of Catholicism to Vodou religions, and suddenly it all makes sense.
Sadly, the wealth-worshiping individualism is not unique to US Christianity and is becomming increasingly popular in Australia through evangelical chuches such as Hillsong and the Family First types. Although they share many characteristics with US evangelicals, their roots are more eclectic (the Hillsongers come from a line of kiwi peadophiles, for example).
I must confess that I don't know a lot about radical shi'ite Islam despite having lived in many shi'ite communities, I guess there just aren't that many radicals about. Unless you mean Ismailites who are one kooky bunch of nutbags (the whole leader demanding his weight in gold thing).
Simon is right. However, this isn't news. As I just commented at his entry, the critique of "American Religion" has been going on since the phenomenon started in the 1840s. And the problem is not violence or hypocrisy, or a distorted emphasis on the Old Testament. Every Christian-dominated nation suffers at times from this kind of triumphalism. What's uniquely "American" is a wealth-worshipping individualism on top of this, combined with a theology of the Millennium dating just from the 1920s which is also uniquely ours. The nearest thing I can think of is radical Shi'ite Islam, if they dropped their asceticism and added Mammon to their mistakes.