Actually, MasterCard and Visa prohibit asking for personal identification when someone is using their cards. AMEX discourages such a practice, but does not currently prohibit it.

For example, in case of MasterCard:
http://global.mastercard.com/consumer/cust_serv.html: The merchant cannot require additional identification as a standard business practice when you use MasterCard for payment.

http://global.mastercard.com/faq.html#c_cust_serv: If a merchant displays a decal with the MasterCard logo to indicate that MasterCard cards are accepted, that merchant can not condition a sale based upon the cardholder disclosing any other information or identification, except under very specific conditions, such as when merchant needs to deliver merchandise to the cardholder's home or business.

There is even a form on the MasterCard's website to report such and other common violations: http://global.mastercard.com/cgi-bin..._template.html