The added tip for large groups in restaurants is rude and I don't support it, but there is a reason behind it. The following has happened to me numerous times when attending big tables :
A party asks for the check, calculates the split amount, and then pays. Most people in the party will leave a tip on top of what they owe except the last one who is convieniently still busy looking for his wallet or talking on the phone. When it's that person's turn, he'll just say : "How much money do you still need ?" So for example instead of paying their USD20 meal, they'll just pay USD5 These guys are just ripping me and their friends off, but there's nothing I can do or say about it. A tip is not mandatory by any means and only belongs to me after the entire table leaves the restaurant.
I thought in France they extracted the tip.