I finally got around to reading the book. I had anticipated that I would disagree with the author (based on many comments here), but found the opposite true. I mistakenly began the book thinking that the author would be, in effect, replacing Israel with America and presenting America (as a whole) as God's chosen people....a "Christian nation." The author dismissed the first concern quickly but I believe his dismissal of the latter was more towards the end of the book. Anyway, there are some conclusions that I found less than convincing, but for the most part I enjoyed the book and agree with the author's conclusions (I was surprised that it was in story format...kinda took me off guard at first).
BTW, his argument was not that Israel has been replaced by America or that Isaiah was a prophesy against America (his connection was that Israel was to be an example to the world and that the OT reflected how God deals with nations and people). In other words, the nature of God is revealed through Scripture. I agree with the author here also.
I may be able to help insofar as the "why." Many believe that God, in Scripture, revealed Himself to man. They believe that while most of this revelation was to and through Israel, Scripture is also applicable to the non-Jew. Some even believe that we are bound by God's moral commandments given to Israel. When they read Scripture they think that they are reading a revelation of God's own nature and of how God deals with people.
They still end up making all of scriptural revelation center around America. Every nation goes through cycles where things are better or worse, but with us, it's "we were blessed in the past, [when our values reigned], and now we're cursed, [and it's always because of everyone else's sins, or some leader we don't like]...", and so it ends up that we're the only country so dealt with like that, because there are other countries that are basically "godless", but still prosper. (And then that woula also assume that all countries suffering are being punished for sin).
If God was really still judging all nations by those promises given to Israel, then all would perish; none would pass; not now, not earlier generations or centuries; and that was the point of the Gospel. Extending these divine "laws" to us today, and then splitting off all the "sin" leading to the "curses" to others is just self-righteousness. It's like "I know I've been good, so it's these other people who are ruining it for me/all of us" (And then, they become the "enemies" people want to arm themselves against).