Honestly, you've been buying WAY too much into the crap the news tells you about hackers and all that. To get someones credit card information the way you're saying, you would have to hangout in the parking lot, wait til someone comes with a laptop, get a wifi-cracker like aircrack, and stay there for around an hour or more until the wifi password is cracked. Even then, you would need much better hacking skills than just using aircrack to get their credit card information.