Amazon theoretically is abusing it's market power by being selective about which products it sells in it's stores. When they removed competing AppleTV/Chromecast products from their own and seller lines, to promote their firestick, that was monopolistic behavior. There's also arguments that Amazon is promoting their own line of products on their platform as well, by replicating products that end up being successful, but this is a new area for antitrust that needs to be sussed out.