Card draining is a scheme in which thieves remove gift cards from stores, capture their numeric codes or swap them out for counterfeit cards, and place the products back on display. When an unsuspecting customer loads money onto a tampered or counterfeit card, criminals access it online and steal the balance.
From the article it sounds like what they’re doing is replacing the activation keys on the cards and then putting them back on the shelf, then when the card is activated what really happens is the money is loaded on the card they have.