Gift cards are associated with money laundering and many online scams. I would guess any usage of them (especially in larger denominations) would attract increased attention and additional risk. That's nonsensical of course, why does Apple sell them if they are also suspicious of them, but I would guess if he had paid with a credit card there would have been no issue.

If you receive them as a gift, use them only in a situation unconnected with your cloud ID, such as to pay for new hardware at an Apple store.