Are you saying that in general terms or is that Walmart's policy? I ask because you actually can do what I said at certain stores under certain circumstances.
It's both common sense and my experience. For instance, last year I purchased 4 Kindles form Target as gifts. The price on the Kindles was ~$80/ea I was given 4 gift cards, each at a value of $15. I was clearly told (and I think the receipt indicated it as well), that returning a Kindle would result in a refund of just $65. Further, the cards had to be used for purchases, not cash value.
Normally, the gift cards are a technique that allows a retailer to put an item "on sale" that has a strict MSRP (such as any Apple product, Bose headphones, Nikon cameras, etc.). In return, the retailer is guaranteeing themselves the revenue through a future use of that gift card. Since, theoretically, the items you will purchase using that gift card will have a margin, it is not costing the retailer the full face value of the gift card.
If you've found a store that will actually refund your money on a gift card that was just "given" to you I would be shocked. Not saying it is impossible, but would be very surprising to me.
Fact is, we're talking about Wal Mart. It's not as if you can't find something (eventually) to spend $75 on!