That's how most stores do this kind of promotion in my experience. Stores that do it a different way are usually the outliers. A lot of people on CAG abuse GS(myself included), but they frequently have the best trade promos of anyone around. Without them the trade/flip economy would be nothing. Also, getting rewarded on top of easy flipping/trading with PUR points/certs is a goldmine. If Bestbuy didn't have such a great system with GCU I'd use GS almost exclusively
Thing is, you would only be able to get these certs if you are a PUR member, so what they are essentially doing is reducing the actual value of your PUR discount benefit. Shady shit