Why go for the grunts... easiest target, and history has proven that especially the US is allergic to bodybags, whatever the cause.So I can see the hate against americans, but I don't see why they go for the grunts. All killing is wrong, and I can't agree with either sides. But both sides definately do have a reason to do what they are doing.
The only reason that there is a big mess south of Kirkuk / Mosul province borders is that in that area there was no working government to take over in the first weeks. The only reasons that there are US troops in the north, where there is a reasonably stable government, are:
1: To keep their promise to NATO partner Turkey that the north doesn't declare it's independence, at least not before Dubya is gone.
2: To give the sunnites a chance to get away before the north declares itself independent (with either Kirkuk or Mosul as capital) with all the obvious property reclaiming that will take place. (there were massive deportations in the late 80's to get the oilfields under pro-saddam control)