I think it's because most people that return the Mac already redeemed the gift card on day-1, or at least applied it to their account, so that cannot be returned.
A little confusing, but not as confusing as the carrier deals from Verizon. They give you a 'free' or 'discounted' phone, but they add charges to your monthly phone bill and somehow take it off. I usually purchase direct through Apple, but I bit once on a trade-in deal with Verizon. I swear I could've done better simply purchasing direct and trading it in with Apple. To this day, I feel like I gave them a perfectly good phone for nothing. I even called Verizon and spoke to a few people. None of them could explain the deal to me.