Some of the unfairness lies in that with the same-ish offerings, Amazon's search will favor the Amazon offerings - FBA instead of 3P shipping, Amazon.com as seller vs third party when the price/shipping is same, etc., but otherwise I think a lot of other things people complain about (cloning products and approaching manufacturers to cut off middlemen) are reasonably OK features meant to benefit customers. As a manufacturer and 3P vendor I kinda made my peace with Amazon on that I guess...
I was one of those sellers and now I'm working on an opensource solution to help sellers migrate and not be dependent on Amazon FBA alone.
If your 3p seller account is an "authorized distributor" of your product, then Amazon has no power to require that your website sell the product for the same or more money.
They are just middlemen. It is a good thing that they are going away.
It results in power prices for consumers.
This doesn't make any sense though, as an argument.
If I am selling to 3rd parties, it is also true that I can set whatever price I want.
So, if I set the price to 5$, no 3rd party will be reselling my product below 5$. So I can still set the price...
Ronald Coase wrote the paper that first got to the heart of this, one of the most important economics papers ever published: https://en.m.wikipedia.org/wiki/The_Nature_of_the_Firm
And all 3p sellers are not middleman. Some are manufacturers and others are simply willing to take a smaller margin or are even selling used or refurbished products that people like me usually buy when given the opportunity to save a lot of money.
On the plus side, 3p vendors contribute by getting a product from 0->1 or 0->100 units per day. This is what the industry calls "prospecting", and confirming that there is product-market fit for some product, and a lot of times marketing it in a way that causes it to sell. Supporting the customers, troubleshooting the various issues along the supply chain... this is something that a 3P seller usually contributes to (since a lot of times the manufacturer is focused on designing and making large quantities of product and not really focused on the rest of the supply chain).
This up-front work basically gets paid over time till it becomes rent-seeking behavior, at which point it gets disrupted by a direct relation between vendor and manufacturer. And basically the marketplace needs to hash out what's "fair" from prospecting to rent-seeking.