This is straightforward government restriction of religious practice and has nothing to do with secularism or the separation of church and state. It's shit like this that gives atheists a bad name.
Secularism simply means that the State does not favour any particular religion or engage in religious displays. It does not extend to banning citizens from such. This decision is grossly illiberal, and I'm disappointed that a country with the long liberal tradition of France is sinking so low.