Seems like use the discounts to attract customers and convince them to join Amazon Prime. The Prime promotions and connection are everywhere in WF, and cashiers are always asking, even when the card-reader has a sticker(s) on it.
For an occasional shopper at WF who is just trying to pay and leave the store, whether or not you have prime the correct answer is "no" to get them to shut up so you can pay and leave.
(disclaimer: I work for amazon, although not WF/retail)