But by doing so they raise the price which prevails in the marketplace to their benefit and at a cost to consumers.
Consider the counterfactual case: if there was an Amazon competitor with higher efficiency they could compete by offering a lower take-rate. Sellers could then sell the same product with the same margin at a lower price, and buyers would benefit from those lower prices.
Instead, Amazon is using its market power to prevent alternative stores from competing with it on price by hamstringing sellers. This means that while the sellers’ margin is exposed to competitive pressure, Amazon’s margin is not. And that means higher prices.