Karma Koin is Coming!

the cards are small and cheap enough that the thefts may go unnoticed.

The thefts if small enough(less then 5 or so cards), probably do go unnoticed. The Retailer doesn't gain much from selling the cards themselves, they undoubtedly get less then a few cents per card sold. The reason they carry them is because its mutually beneficial, the overhead on the cards is nothing and people come in to buy a popular item. I could go into possible ways the "hacker" can activate the cards at home, none of which are really "hacking".