If a card is invalid (i.e. no signature), the cashier can ask for identification to verify your identity, and watch you sign. If the card is already signed, cashiers may not ask for ID as a condition of the sale, the transaction is considered valid if the signature on the card matches the signature of the receipt.
If your dad had simply signed the card prior to walking up to the counter, that's all the cashier would have been able to verify. So yes, card security is rather lax if it's not signed by the owner of the card immediately upon receipt.
I know this random information because I was a bit miffed that Walmart rejected my girlfriend's transaction because she did not produce ID with a signed card. The above applies to the Visa merchant agreement, I'm not sure about Mastercard or American Express.
Also -- "SEE ID" is not a valid signature, though you can write that over your signature and the card is valid. However, cashiers are not required to honor it.
> the transaction is considered valid if the
> signature on the card matches the signature
> of the receipt
In the example above, both the receipt and the card were signed at the register. It would be extremely odd if they didn't match, even if the person paying was a fraudster, therefore it's odd for her to verify them.