That's not how it works, and I have firsthand experience, and hasn't been for at least three decades. The very first thing that happens when you show up in Fontenay-sous-bois is that they run Interpol checks and other checks in your country of residence or anything notable in your passport.

Gone are the days of accepting any criminal openly.

They do however, if you either complete your contract, or are injured, deem you eligible for French citizenship "by spilled blood".