Are 3rd party sellers entitled to fairness? They choose to use Amazon as a platform, they could use ebay, etsy, woocommerce, shopify, bigcommerce? There is also not much but complexity and time stopping them from using a bunch of those services. Theres also things like ecomdash.com that let you treat all the platforms as just another sales channel of your business.
Im not sure how much I feel for the "I sold my soul to the devil for a quick buck and he turned on me, woe is me" crowd. If you dont like the deal, and you dont want to list your product at one of the biggest stores in the world, nobody is forcing you to.
There may absolutely be some anti-competitive behavior happening. Im not sure how I feel about amazon watching trends and then cloning products. Being paid by their competitors to be fed market research. But its not a unique business model to Amazon, Wal-mart has been doing it forever. If this behavior type needs correcting, correct the rules, not haphazard sanctions after the fact.