I have no problem tipping in the moment. And sometimes for very different amounts than expected. But if you expect a tip and didn't earn it...?
Tips are absolutely not mandatory in France. You pay for what you order at the price you see on the menu and nothing more. Small tips are appreciated but not expected. I am French, I have eaten in many restaurants of all kinds and I have never been asked to tip and I tip only occasionally. As for surcharges, they sometimes happen, for example if the restaurant is part of a club and you are not a member, but never at regular restaurants, it may even be illegal.
I am sorry for your experience, they probably noticed that you weren't a native and attempted to trick you. Was it in Paris? It seems like such unacceptable behavior is more common in Paris.
Note: I interpreted "mandatory tip" as having to pay a surcharge. Of course, part of the bill goes into paying the waiting staff, but it shouldn't appear on the bill, that part is between the employees and their boss.