I just took a look at the letter that came with the card, and it begins: "You're one of our most valued online customers and I want to take a moment to thank you for shopping at GuitarCenter.com..."
Obviously it's a threshold of dollars spent online that qualifies a customer for the card.
There is no minimum purchase toward which the $25 is applicable. Just free stuff, which is hard to pass up. I expect the store will be packed when I go in this afternoon since there it's only valid this weekend.
Corporate can probably write it off as marketing expense and sales will surely spike above and beyond the layout.
Smart marketing move, I think.