I've lived in Paris for the last decade. To me the only surprising thing about this event is that it didn't happen sooner and it wasn't much worse.
There are substantial Arab, Jewish and African black populations in the city and many of your average (white) French don't like it. The fact is that racial tensions here are very real and fairly widespread. There's no doubt that the radicals have behaved monstrously but a lot of French people (at least here in Paris) think it's quite entertaining to make disgusting jokes and racial slurs about non-white French, among others, not to mention flaunting outright racial hatred as if it were something to be proud of. At the end of the day a non-trivial percentage of French behave as if most (if not all) "foreigners" are filthy, unwanted invaders and the cause of all their problems. Sure, we've all heard about this kind of crap before but here it truly is a part of life, just part of the current running through the city and the country at large.
Look to the steady rise of Le Pen and the racist right for some perspective on the problem. Also look for stuff on Youtube for the kind of thing that goes on in the Paris suburbs, the banlieu, where the population is disproportionatly non-white and immigrant and where unemplyment runs as high as 20%. This New Years for instance the news reported that it was "a quiet year, only 98 cars were torched". 98! Anywhere else that would be reported as a major riot but here it's just routine. Examples of these tensions pop up all over the place: in my neighbourhood, for example, a Jewish kid was kicked to death a few years ago by a gang of French and Arab high schoolers. In the offices my wife works in as a consultant staff will often tell her, in confidence of course, that this or that social ill is the fault of "le petit noir" ("the little blacks") or the Arabs or whomever. These are middle-class or upper-middle-class professionals! It's not poverty or social injustice driving their racism, not that those are excuses either.
And that racism isn't limited to Arabs or Jews or African blacks which, of course, is bad enough in itself. My wife and I are as white as it gets and we get a fair amount of it ourselves, from all sides. For me it's just anti-foreigner stuff but for her the verbal abuse is often sexual which makes it doubly hard to deal with. Either way it's not fun and it never stops. We've been watching the situation worsen for years and have been fully expecting something, somewhere to blow.
This is why I say the recent attack isn't much of a surprise: to us it seems more-or-less to be expected because the root causes of that kind of thing are everywhere, every day here. And the French government is no help. They are nowhere to be seen: the general approach seems to be to leave it to people on the street to sort it out for themselves. In an economically difficult environment challenged with rising unemplyment and laced with racial tension that seems a particularly short-sighted approach to the problem.