They ARE able to give discount codes for ALL items, they just use MAP as a reason not to. Minimum Advertised Price only means they can't openly display prices that are lower than the Manufacturer has set, however, they are able to do "See Price In Cart" and have a much lower listed price. Manufacturers cannot control what a business sells items for, businesses have already purchased the items from the manufacturer, and if the business wants to take a loss on it, or shrink their profit margin per item, they are allowed to do so.
Edit:
@Gtinnel totally did not mean to sound like I am yelling at you, this is just my single most frustrating issue with BRS because it's dishonest for them to blame it on MAP, and people should be aware, if enough of us know and take issue with it, they might re-think their strategy and open up about the real reason they don't discount MAP items, which could be anything from greed, or another form of contractual obligation unrelated to MAP that consumers should know about before deciding if they want to purchase from a certain manufacturer or not.