wow considering how long this has gone on, there must have been a handful of people who made a lot of money doing this. A starbucks giftcard is almost like a currency . I wonder if walmart cards have a similar vulnerability
I can only assume this is because gift cards are intended to behave like cash (when the balance is zero, they have no additional value), but operate under the mechanics of credit/debit cards (asynchronous transactions, network failures/delays, etc.). Debit cards have the feature of going negative with very clear legal liability assigned to the account holder. Gift cards holders are essentially anonymous, so recourse may be more difficult.
One of their first concerns should be is any facet of this affected by race conditions? It's not that complicated of a problem really..