The right question, I think, is not whether religion has an undue influence, but why it is that the current flourishing of religious faith has, for the first time ever, _virtually no element of social justice_? Why is its public phase so exclusively focused on issues of private and personal behavior? Is this caused by trends in the nature of religious worship itself? Is it a displacement of economic or social pressures? Will that change? What are the factors that might cause it to change?
I don't think this is quite the right question, although it is close. Having attended a Catholic college for my undergraduate I heard practically nothing but social justice for the three years I was there. The author is explicitly thinking of American Protestantism; but I'm not sure that this time around we can make a sharp distinction between Protestants and Catholics in terms of the flourishing of religious faith. Catholics, to be sure, talk a great deal more about social justice than Protestants do; but they are at least there as a sign that it is not going on without an element of social justice - the question is not why social justice is not there, the question is why so many Protestants are missing out on it.
It is worth noting, incidentally, that several of the trackbacks to the author's post (or to the derivative post at Crooked Timber) ignore the fact that he is talking about Protestantism, and treat it as a general question about 'the religious right' or 'Christianity'. This would make it a nonsense question.
It is interesting to look at the Crooked Timber post, though, and realize that whatever the CT folks mean by it, they mean something much, much, much narrower than the comprehensive Catholic project, which includes, and has included since Rosmini and others first introduced the term 'social justice', state defense of human rights, universal Christian charity, and moral economy as its main elements. The reason why conservative Catholics are becoming so vehemently pro-life in their politics is that they see it as a social justice issue - like the restriction or abolition of the death penalty, the living wage, and the like. It is a cause, I think, of some recent shifts in Catholic support from the Democrats to the Republicans: it is a very reluctant shift, forced by the fact that many Catholics are having a harder time seeing the Democratic party as standing for the most important elements of social justice - so they end up voting Republican on the basis of particular social justice issues they think important, but which they have difficulty finding with the Democrats.