If they've added 15%, regardless of what they call it, "service charge," or other, it seems reasonable to give them another 5% to 10%, unless the service was horrendous. People should expect to pay at the very least 15% when they are in a restaurant. If you leave no tip at all, in a place where there is no built-in 15%, and you do so because of poor service, I think that's just as douchey a move as a waiter giving you that poor service to begin with.
Just as "duuchey" as starting with an incorrect premise, then calling names? You are applying a US standard to the rest of the world. Europe has a very different salary / gratuity system than the US. And here, you are on a European island, not a US island. For the record, I am born, raised, proud to be an American, but really get tired of some of my countrymen thinking the world revolves around our way of doing things. It is a big world out there and one size does not fit all! (end of rant)