The difference is that you are the member and you were the one who was actively making payment using card from your wallet and entering a valid pin.

The cashier has no reason to question the transaction under those circumstances.    But your scenario goes to show that there is likely a little bit more to this story than being presented.  Odds are a manager on duty likely would have approved the transaction if the author had dealt with the situation in a less hostile manner.  In fact I have witnessed cashiers and supervisors over ride store policies while politely reminding the customer of the rules and to not expect the courtesy from other cashiers or in the future. 

Your father's bank / credit card company on the other hand may not be so happy with you having the card and pin and may not be so supportive in the event your dad's card has fraudulent transactions posted.    Don't get me wrong I have done similar things with family - but just because we can doesn't make it always right.