Amazon's brand (as a store / marketplace) is that it helps you sort through many options to select the product they believe you will like the best. This is different from the service offered by traditional grocery stores which offer many different products, but are not arranging those products in relation to a particular request.
This means that, when Amazon is operating as a store, any re-ordering of the display results disrupts the value they provide to consumers. It also, of course, disrupts the value they provide to the products they sell. Any other product that receives poor reviews would be punished by being displayed later, but Amazon branded products will never suffer that fate. The same is true of any other algorithmic sorting.
Amazon is, with one side of its mouth, suggesting a business model that it claims will benefit both sellers and buyers (that the best products will win and consumers will be shown the product best matched to them) and also explicitly excluding itself from that system.
To return to the supermarket situation, you might imagine a supermarket which stocks all brands, but the first item in every row is the store-brand equivalent of that product which you must remove to get to the branded one.