Their gift cards ($100 for $40, etc.) obviously can't be used to purchase additional gift cards; they're only redeemable for restaurant vouchers.

Otherwise, this would be insanely abused even beyond miles/cashback referral offers -- you could simply buy a $100 gift card for $40, use that to buy 2 $100 gift cards for $80, use that to buy 4 $100 gift cards for $160, and so on and so forth.