I'm honestly intrigued as to how this is so morally unambiguous.
Walmart, any-given-supermarket, Costco, etc., have in-house brands (for example Kirkland with Costco). These became prominent in the 90's and there was similar discussion at the time about the ethics of a supermarket introducing competitors to the brands it stocks. Such controversy has died down and now it is commonplace. Looking at some updated articles, in-house brands at those retailers have in many (most?) cases outsold their branded competitors, so in-house brands like Kirkland are actually the largest brands in the country.
So my question is: what makes Amazon different, behaviorally, than Costco, Walmart, Vons, etc.? Both use an (arguably) unfair data asymmetry to introduce in-house branded products to compete with the external brands they also sell. You then have brands like Trader Joe's that go out of their way to have an in-house brand, and make few exceptions.
On open question is: who is this behavior actually hostile to? I think it's now conventional wisdom that if you're a brand selling in Vons or Trader Joe's that you need some special sauce to compete successfully with an inevitably-introduced in-house brand. So what is that special sauce? You have some brands that are exclusive because of their provenance or method of creation (say, a particular Scotch). But then you have many brands that are actually manufactured in the same locations and by the same people as the in-house brands, wherein the brand itself is a middleman who created a particular formulation or price/materials ratio that may or may not be the best one. These would seem to be the most vulnerable to Amazon building an in-house competitor, or Walmart, or Vons, etc.
I suppose, looking for the other side of the argument, that the question would be if Amazon has so much monopolistic market clout that the third party has no way to exist once the in house brand has taken over. E.g. as though Costco were the only supermarket period, or, more accurately, if Trader Joe's were the only supermarket period.