The difference between Amazon and traditional retailers with shelf space a la grocery stores is that grocery stores pay wholesale for the 3rd party products they put on their shelves. So when you see generic fexofenadine sitting next to name-brand Allegra in CVS, Sanofi already received payment for the name-brand bottles (or at least a commitment to pay, depending on net terms). The risk of selling them to consumers now falls on the store.

Amazon does not pay wholesale for items in its marketplace. In fact the marketplace was explicitly set up to be a logistics-only service and level playing field for anyone who joined. Now it's not, of course. Basically they pulled a bait-and-switch on their marketplace customers.

(Ironically, they also pulled a bait-and-switch on their first retail partners by signing exclusivity agreements, and then breaking them when they launched marketplace.)

This is also why Amazon struggles to fight counterfeit products. For most of the products you can buy on amazon.com, Amazon the company does not have buyers who can verify product quality and supply chain before placing a wholesale order. It's a logistical struggle made worse by a cultural struggle, i.e. they don't want to sell counterfeits but they also don't want to do any of the normal work that retailers do to prevent counterfeits.