Maybe they don't want to use troops as an occupying force. Afghanistan / Iraq already proved how difficult it is to occupy territory, it'd be a lot easier without insurgents resisting. A war of attrition would suck up a lot of blood and treasure the government would need to ward off threats to a much-weakened U.S.
Maybe the half life of radioactive isotopes is acceptable enough to relax habitation restrictions in a short enough period of time. People do currently live within the restricted zone of Chernobyl, they just have shortened lifespans, but they are functioning properly. You can probably garrison troops after nuking an area six months after doing so, as long as you cycle them out on a regular basis.
Maybe white-collar service workers are less valuable than blue-collar laborers, especially if they're already demoralized. You can torture a blue-collar worker to create output, albeit slower and lower quality and with Byzantine faults. You can't torture an asset management consultant to give you a high ROI. The brain doesn't work like that. So at that point he or she is just another mouth to feed and you can get rid of him or her. Extrapolate to an entire city.
Maybe it's just pure hatred. Most cities are highly Democratic and liberal, which today apparently labels someone as less than yourself, as someone less than human.