> it uses its access to their sales data to come up with its own competing products.
Every retail chain has data on what gets sold inside and where it was kept, how it was promoted etc. They can very well use this data and most probably are using it.
> Thus, it is irrelevant to the manufacturer whether the supermarket promotes their store-brand product or the name brand product;
Only till the time when their store brand starts selling much better and consumers develop a relationship with that.
> But in the case of Amazon, the third-party sellers on Amazon's website get screwed, and that is where the antitrust concern lies.
Isn't this very similar to the third party sellers having their own Shopify store and someone discovering it through Google Search/FB Shop instead of Amazon Search. The only argument that makes sense here is the usage of data.