Foschini trading discount for your ID no

I was astounded having not bought from Foschini for a long period. Yesterday I bought some items cash. Upon paying, the cashier informed that there is a 40% discount on the items. She needed my Id no. I enquired why a company would want your Id no in order to trade it for discount. She could not answer me. I received an urgent phone in the interim and she proceeded with the transaction without the discount. My question is still why does a company want your ID no for a cash transaction. I feel it is against FICA to obtain personal info on a client that has nothing to do with the company.