Gift card nightmare

I was very disappointed to learn that everything in the store can be bought with a gift Voucher expect for a form. Like how, aren't the phones sold by the store aswell. The disappointing part is that, this was a gift for my son who did very well in his Matric, only for him to get to the shop and be told he can buy anything except a phone, like what the hell. Then what's the point of the gift card. When we asked to exchange it for the cash, so he can buy the phone, they bluntly refused. I really need someone to explain this to me cause right now, I do not understand.