In Los Angeles, I one time tipped someone like 10% of everything. But then I got insults after from the waitress who brought us drinks all night. I did not understand what I did wrong. But someone explained that if I write the tip amount in the receipt and let the tip amount take out of my card, then it won't go to the servers, but straight to the owner of the place instead. I had no idea that was the case.
They should understand that those who come from countries, where they don't have tipping tradition, don't know how it works....