This is because: a. Almost all US merchants take all VISA or MC branded cards, while in Europe many vendors will only take debit cards because of lower fees.
b. The USA doesn't have chip-and-PIN, so it's laughably easy to use a stolen bank card. At least now with EMV and signature, someone has to actually go through the trouble of stealing your card instead of just cloning it. But there are still a ton of merchants that are just using mag swipe ...