Regarding the Zones Urbaines Sensibles:
I just checked, out of interest, which those are for Grenoble (where I lived for the past four years) and it's neighboring communities. I found around half a dozen. Two of those I checked out closer, and they were places I went past rather frequently.
Since I had no reason to enter these blocks, I didn't try to, though maybe I once cut across one on my bike. Anyway, these are the usual ghetto 60's/70's immigrants and poor people, living where housing is cheap and run-down. No particular muslim slant that I could point at, but probably a higher than average crime rate. Grenoble had some rioting in early 2010, just before I moved there, and even while I was there I saw the odd burnt-out car in town. Also, the mayor sent everyone a nice letter pointing out how the town had been assigned priority national security status, assuring the support of the local police force by further national police forces. And yet, none of this was religious tension, most of it was due to the high amount of organized crime, and immigrations and social issues.
I assume that in bigger cities, such as Lyon or of course Paris the likelihood of Muslim fanatics organizing at a larger scale is higher, and as such some of the hotspots marked as ZUS in those regions might well be due to religious issues - but they are designated with the goal of remedial action and focused security and enforcement actions. Not with any goal of letting them run themselves, or leaving policing to the locals.
There are places, where police obivously will not head without back-up. But probably no worse than this same situation would be in Detroit.