Let me see if I understand the laws on the french side correctly:

1) If the menu handed to you, does not have a notation " service non comprise", Then the assumption is service is included in the menu prices and no additional service charge can be added at the end to the bill. So if the total menu items is €100, the final bill should be €100 as well. And you may choose to leave few € in change.

2) If the menu handed to you, does not have a notation " service non comprise", Then the assumption is service is included in the menu prices and no additional service charge can be added at the end to the bill. So if the total menu items is €100, the final bill should be €100 as well. However if you are handed a bill of €100 with stamped " service non comprise". It appears that the establishment is being greedy trying to pull a fast one and hoping you will leave additional amount over legally included service charge

3) If the menu handed to you, has a notation " service non comprise", Then the service charge can be added to the bill, but not as a separate item. So if the total menu items is €100, the final bill could be be €115 ( assuming a 15% service charge). In this case then the total amount of €115 can be charged to CC.
However the bill should not be presented as €100 food drink line, with a €15 service charge line with a total of €115 line.

4) If the menu handed to you, has a notation " service non comprise", Then the service charge can be added to the bill, but not as a separate item. So if the total menu items is €100, the final bill could be be €115 ( assuming a 15% service charge). However even with that if your bill is stamped " service non comprise". It appears that the establishment is being greedy trying to pull a fast one and hoping you will leave additional amount over €115.

I remember few years back (almost 6 years ago) one of the GC establishment ( if my memory serves me right, was La Villa) stamped "service not included" on the final bill, if they felt you were an American. We were warned ahead of time. We went there twice that year (12/2010). First time with bunch of Spanish and french speaking friends. There was no stamp on the final bill. The second time, it was just the 2 of us and at the end we were handed a English bill with "service not included stamp" and that that part can not be charged on the CC. I just felt they were trying to take advantage of the Americans.

Nick