Amazon's perspective is that they are a store / marketplace that a seller can optionally sell on. They are ensuring their customers get the best price. Like any store, if you don't like their terms then don't sell your product in their store. Pretty solid logic I think.

But the reality is they are a store that dwarfs every other in customer reach making it difficult for small businesses to grow without utilizing Amazon as one of their sales channel. Hence, Amazon's 15% commission gets baked into everything even if it's not sold on Amazon. I think the law needs to evolve in a way to recognize scenarios like this where there is massive asymmetry - not necessarily a monopoly - between one dominant market player and others which is harming the consumer.

Amazon's response of course would be "no fair; you're using us for product discovery but then giving the sale to the brand owner". Which has some validity. If you took away the commission, then they would be relegated to a search engine that relies on ad revenue only. But that would mean lower prices so consumers would win in the end.