they are not allowed to import any and all aspects of their culture willy-nilly, and it's not racist to want it that way, imo, though many will surely howl that it is.

The re-islamisation of French citizens of maghrebo-islamic origin is not a matter of re-importing North African customs into France, but largely a matter of importing stylised versions of 9th-century Arabian Peninsula customs. The niqab, for example, is not a North African custom, and its recent introduction into North Africa is the subject of a great deal of social conflict (see Tunisian universities, for example). The Christian equivalent might be for Roman Catholics to start dressing like Romans. Wearing a niqab in public is inherently ridiculous in Europe, and it is neither racist nor religious persecution to ridicule it.

it can be patiently explained that they are guests here and have to respect that

No they are not guests : in their vast majority they are French by birth, and are completely free out of republican principle to practice their inherited or chosen religion as they see fit. What they have to respectis French law and secular custom (laïcité). In practice, there are many obstacles : it's exceedingly difficult to get a permit to create a mosque in France, and I deplore that. (There is, to my mind, a large surplus of Catholic places of worship with respect to effective demand, but every time I suggest that some should become mosques, I get funny looks.)

_{It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II}