The type of contract you are describing is common in many industries. They’re known as “full content agreements” in the airline industry for example, and they’re a continual source of tension between the distribution networks and travel agencies (who want these rules) and the airlines (who would like to be able to sell cheaper on their own direct channels). The same thing exists for hotels etc

I guess the question boils down to whether Amazon can be considered to have a monopoly position in the indirect distribution of the specific type of goods being considered. I imagine that might be possible for books, not sure about other goods