[mikeash]

Why do people care so much about guarding their personal credit card details?

In the US, at least, there is zero liability to the cardholder for fraudulent purchases made without the cardholder's signature, by law. Reporting fraud is fairly easy, and getting a new card after your details have been stolen is free and takes just a few minutes on the phone. You're without your card for a few days while it works its way through the postal system, but that's why you have multiple credit cards.

Companies need to care about this a great deal because they're potentially liable for a lot in case of problems. But individuals have no real reason to care about the secrecy their own card details. Yet, people are constantly worried about it anyway. Why?