A lot of the beef with Amazon seems to be "welcome to retail". I don't think Walmart even has third-party logistics for its stores -- or if it does it's of the "hire contractors and squeeze them hard" variety. Personally I've found the customer experience of non-FBA merchants on amazon is often pretty bad: long shipping times and weird return policies. This gets at a sort of core problem with this: where is the line…
What they could argue is about the myriad of ways that Amazon uses its market power to put others out of business or force them to sell (diapers.com comes to mind on this[0]).
[0]: https://slate.com/technology/2013/10/amazon-book-how-jeff-be...