This is because they are considered a discount or rebate. You pay with a credit card and receive a small percentage back. The same applies no matter what form your rewards take.

They look like gift cards, but they're not. Often too late, people discover the difference. Unlike gift cards, which are regulated by federal law, these promotional and rebate reward cards can ...

So, I bought a virtual $15 Amazon gift card, sent it to my own email, and then used it to buy a $19 computer mouse with my actual credit card covering the difference. This way, no money is left ...