I can.

There is no tip line based on the way the credit card system (and the service charge system for the government revenue) is set up. One would have to change the entire system, just to meet North American tipping desires. (There is technically a way to do it but it is more complex, has to be requested prior to the total being entered, and is not something offered as a solution due to the time and cost factor, and may never reach the server)

That's why one is advised that if one wishes to tip the server, to do it directly with cash. And I do believe that is the advice that some of gave that poster in past, about tipping in France.

I believe that these are the different steps (I included Canada since that poster is Canadian and maybe was confused when he was given a credit card machine for his chip/pin card, but no tip option)

In the US:
- server brings bill to table
- customer fills out tip line
- server takes bill back to machine, enters total with tip
- server brings bill back to customer to sign

In Canada:
- server brings bill and credit card machine to table
- customer enters their chip/pin card and machine asks if they wish to add tip
- customer enters 'yes', and enters a percentage or flat amount
- machine produces receipt based on total plus tip

In France:
- server brings bill and credit card machine to table
- total is entered into the machine
- customer enters their chip/pin card into the machine
- there is no option for customer to select 'yes' to add tip, unlike in Canada

(But I do agree with you that the defense of tipping/over-tipping has been rather, er, 'interesting')