This is fine, good for Amazon for setting up an excellent logistics system and having a good return policy. The problem is that Amazon charges a lot for this service. If you don't need super fast shipping or a good return policy, or don't want to pay for Amazon's profit margin, you can't go anywhere else, because Amazon will restrict your sales if you charge less somewhere else.

Given that Amazon has 40% of the e-commerce market (I'm surprised it's not more actually), this is exactly what antitrust laws were designed to stop. Customers and vendors/manufacturers should be free to use the distribution channels that meet their needs of service and speed and price the best, but Amazon, through its big market share, is preventing that.