The important thing to remember is that, in essence, scammers are operating on their home turf. They get to dictate the payment method, and as a result a (good) scammer will make sure and use a "payment method" that meets the following two criteria:
- The transaction cannot be reversed
- The payment can easily be converted to cash or used directly
Note that the above criteria are why Western Union is so common in scams: once you send the money you can't ask Western Union for a refund a week later, and on the other end the scammer can receive cash directly. Gift certificates have the same advantages:
- You generally can't cancel the purchase of a gift certificate
- A scammer can use a gift certificate to buy stolen goods for sale, can sell the gift certificate directly, or even just use it to purchase things they needed anyway (h/t MechMK1)
It's possible that some kinds of gift certificates may be cancelable. It is safe to assume that the scammer knows about the exceptions and avoids them. After all this is how they make money, and in a properly executed scam the victim is going to follow orders. Therefore the scammer will choose whatever is most convenient for them.
It's also important to keep in mind that a scammer may be working more than one "angle" at the same time. For instance a scammer who gains access to a bank account doesn't just send the money to themselves. Instead they may transfer the money to another victim who sends the money to the scammer via Western Union/gift certificates. This way when the bank rolls back the bank transfer (because bank transfers are reversible), someone else is left holding the bag. Similarly a gift certificate may be sold/used quickly or exchanged for cash with another victim and as a result the scammer doesn't care if it is cancelled.
Still, if I were a betting man I would just assume that the scammer has requested payment via non-refundable methods.