Spaniard here. FWIW, my first experience eating in a restaurant in the US during a trip went as this — I got the bill and gave them my credit card to pay. A few minutes later one person from the restaurant came to me and told me that I had forgotten to write the part of the tip in the bill; he came to me because he deduced from my credit card that I was European and was not aware of the customary procedures in the US.
I was indeed not aware of the procedures (I thought it was like at home; “service included”) and wrote a tip in the bill. But it left a bit of a sour aftertaste.
From then on, whenever I have been at a restaurant in the US, I have made sure to leave a tip. I still find this “quasi-mandatory” tip weird, but I guess that it is a “when in Rome, do as the Romans do”.