I think they ask if you want to add to the check on credit card because they have to put in the exact amount into their little machine.
As opposed to entering the tip later.
Should never be asked in the first place.
We've been asked the same question at restaurants on the French side. While I agree they shouldn't ask, life's too short to stress over small stuff. We simply respond that we'll leave cash, and leave a small amount, usually 5% or so. Sadly, tourists who don't understand that the service is included on the French side, will leave more. Just another reason to learn the customs of the country you're visiting.
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