Bad customer service Id number is not sharable

I was at fou rwa truworths they are running a promo, I went to the till to make a payment the supervisor or the manager said we gona need ur phone number so that u can get the promotion but the cashier also asked for my id number why can't they write when they do the advert that they're promotion needs id numbers even if u not opening the account why id number if I do cash payment?