There is no good reason to believe in multiculturalism where islam is concerned.
France's opposition to multiculturalism means that a Frenchman is a Frenchman is a Frenchman. Doesn't matter if they are an Algerian Muslim Frenchman or a white Christian whose ancestors can be traced to France for centuries. The French census doesn't even count for ethnicity.
This is a problem with travel. Many countries, including ours, have very lenient policies for granting travel visas to French citizens. If the French don't categorize ethnicity, it could potentially become difficult for other countries with liberal travel arrangements to keep out these so-called "French" terrorists.