At the point that greedy corporate vultures realized that people don't perceive or completely understand the privacy you're giving up in exchange for a few percent off your order, which you better believe has already been jacked up to compensate for the discount.
GrapheneOS protects your privacy, so if the companies pushing their apps can't harvest your data, they see no reason to let you use the app to get your discount.
If they can't trick you, they don't want you playing their game.