I don't know why...
Artscroll issues GCs for 2 types of people:
1. Those that pay full and don't want to just give cash, such as people giving a Bar Mitzva Bochur a gift, and
2. Organizations or the like that don't pay face for the GC, and raffle it off, or Chinese Auction it off etc.
I would assume that knowing that there are sales at some point in the year for 20% off or 33% off or whatever, Artscroll is willing to sell the giftcards to Mosdos at a percentage off, providing that the GC can't be used on sales. They probably view it as giving the sale price to the Mosad anytime throughout the year. Combine that with the fact that they get the cash upfront....
Either way, I wouldn't assume that its really "Good as cash", unless you actually spoke to Artscroll and have some way to prove it.