"Granted, European countries often exacerbate this problem by failing to thoroughly assimilate their immigrants in the first place. The isolated North African ghettos of Paris are the perfect incubators for radical Wahabbism, even if they are mostly moderate now."
This is what progressives mean when they say don't promote foreign policy that will provote muslims. In this calculus, so called moderate muslim "yutes" will only embrace radical Islam when they have been sufficiently threatened and offended; they have no free will to take Western response in context, and thus they cannot be held responsible for becoming extreme.
They're right.
Liberals don't believe in assimiliation; they believe in multi-culturalism. People have categories they must be lumped into, and they are not permitted to change them simply because they desire to. This stems from a misguided desire to put people and cultures onto an 'endangered species list' status where they are protected, and come to rely on liberals for that protection. You can't have victims if everyone's on the same team, and liberalism needs victimhood to surive.
Unfortunately, in the real world, victims aren't helpless. In the rest of the world, victims can easily become oppressors. It happens all the time. When you keep the flames of discontent burning in people because it helps your political standing, you run the risk of losing control of that fire.
Now, in France's case, it appears that good old fashioned racist/elitist snobbery had a large hand in keeping the Muslims segregated. Still, the bottom line is that the French chose not to force their new immigrants to assimilate, and they now have a growing segment of their population that feels alienated and victimized. A segement of the population, incidentally, that hails from a culture where brutal acts of revenge are commonplace.