Not sure why this is voted down. Amazon is just doing a scaled up version of what Walmart, Price Chopper, Trader Joe's and various other supermarkets have done for decades. Why should Steam and Amazon be barred from doing it if others aren't?
Additionally, Trader Joes ONLY sells white-labelled products, so it is even less applicable than your other examples.
Their mix is heavily skewed toward private-label products, but "ONLY" is not accurate.
To pick one example: they sell name-brand beer alongside their private labels.
In most cases, a market (in the specific sense, like a store or a site) will white label a product and undercut competitors by leveraging the volume/wholesale pricing that they can get. What's truly dangerous about the AMZNs and GOOGs (and Standard Oils and Ma Bells) of the world is that they can use brute monetary force to buy or subsume competition. To quote the legendary SF grifter Jack Black, "You can't win."