True, but if the corporate office notices that one store almost never has any reviews posted, or notices that their sponsored site (with the receipt codes) has

True, but if the corporate office notices that one store almost never has any reviews posted, or notices that their sponsored site (with the receipt codes) has a store getting 5-star reviews (indicating that the receipts are only being given out to people likely to be positive), while a non-sponsored site has markedly lower scores, then you'd probably look into what's going on there. And the best way to figure out how a store is actually doing is still to go there and see for yourself.