That might be true if Safeway is going to buy the same number of cereal boxes from you every quarter, but that's not what happens. Stores adjust their purchases based on what sells, so if Safeway starts promoting their own cereal then they will start buying less of yours.
Notably, and very importantly: Amazon does not pay for the third-party sellers products available on its website, the third-party sellers have to pay that.
Supermarkets frequently charge slotting fees to appear on their shelves.
The manufacturers get paid either way (by the retailer, third-party seller, or Amazon if Amazon.com is the seller). But in the case of Amazon, the third-party sellers on Amazon's website get screwed, and that is where the antitrust concern lies.
Supermarkets often have contracts where they can return stock to suppliers if it is defective or not selling well and do not always own their stock.