This is the only relevant comment in this entire thread. What's more, the 3-digit "security" code (pardon me, whilst I scoff) for these cards is printed on the

This is the only relevant comment in this entire thread. What's more, the 3-digit "security" code (pardon me, whilst I scoff) for these cards is printed on the card! If you hand someone your card, you're already saying that you trust them not to keep your card number and security code and submit false charges. The Square reader isn't making that process any harder or easier; it's totally irrelevant.