I actually suspect this is not simple Apple blockheadedness. A gift card represents money already spent. If the user buys something with that card, no additional money goes to Apple, but they do have to pay out money to a publisher. So Apple has a vested interest in preventing you from spending your gift card, instead resorting to further cash payments. The longer you are thwarted from using your gift card, the longer they can hold onto the money without paying publishers.