It is more about preventing "double dipping" I think than collecting money. There was a vendor I use regularly through the portals that gave cashback when purchasing their gift cards AND when you purchased with their gift cards. So I'd buy a GC for my purchase (round 1 of points) and then make purchase with said gift card (round two of points). They finally closed the loophole and now you only earn points when purchasing the GC not spending it.