I think their point is simply that "it's their store" - and to be brutally honest, I agree.
Two not-entirely-random examples of similar behaviour:
Supermarkets place their own brand merchandise where they feel it'll sell best. Undeniably good business, and certainly no reason for concern.
Perhaps more questionably, but bare with me - Google place AMP content, which is to say content they have made their product by merit of some ToS/caching slight of hand, front and center. Is it exactly the same as what Amazon are doing? Well, no. Is it placing content they want consumed above other content irrespective of merit or consumer benefit? Probably.
Similarly Amazon are doing what businesses do and promoting their most profitable products, straddling both the above examples. Whether it's best for the customers is open to debate, but it doesn't change that it's entirety their prerogative.
Yeah, the search vs shop tension is the big question.
The difference being that Google are ostensibly a search engine, who are looking to monetize and leverage their ubiquity - Amazon are a book store grown to titanic proportions looking to maximize profits.
The same, but different.
I fully support Amazon's divergent empire being broken up into sperate companies, but for the moment Amazon.com - the online marketplace - is still an internet shopfront.
