It would only be wrong if you bought the gift card, kept the promo, and then tried to get a refund on the gift card.
Which, yes, many people actually do try.
It is not your responsibility to guess someone’s intentions. An unwritten rule is something that society follows and will punish you for if you don’t follow it. It’s not guessing what the other person wants.
And I do not agree with the application of Kantian ethics used here. This is a situation where the percentage of people who will take advantage of the deal is built into the offer. It is entirely expected and intended that some customers will buy the cards and use the deal, or else you wouldn’t offer the deal.
It’s not like loss prevention, where they would rather no one steal but can’t 100% prevent it.