My guess is that gift card usage is bimodal:

1. The large set of gift card transactions that involve just one or two gift cards being used in a long period of time by a single person, which don't set off fraud flags.

2. The other large set of bulk gift card uses, which are overwhelmingly fraudulent.

So the answer would then be: they're sold for case (1), and policed for case (2). Don't do case (2).