Observing that Amazon may be monopolizing isn't a new observation.
When they began competing against their vendors is when they crossed the line, in my opinion. The same goes for any marketplace. It is one thing to provide your vendors the tools and information they need to operate better, but then to act on that information yourself - the operator of the marketplace - that is pretty shady.
In fact customers are already trained for this. I walk into a store and I immediately price compare against the store brand. So if people doing the same on Amazon, why shouldn't Amazon be allowed to fulfill that? This was a thing in retailers a long time before Amazon started doing it.
If you look around in retail stores today its easy to see how everyone strong arms vendors just the same. For example, ever notice that all of Target's pen section all have a certain color and text printed on the bottom of their packaging indicating the type of pen (Gel, ballpoint, etc)? Target has forced every single vendor (massive companies like Bic, Sharpie, etc.) that sells writing implements in Target to change their packaging, create new SKUs, do different production runs, and handle the logistics of different versions of the same pen. All to be undercut in price by the Target brand pen.
Antitrust law used to address this. Monopolies are not allowed to do things that non-monopolies are allowed to do, because the effects of monopolies doing them are different than the effects of non-monopolies doing it.
Of course, Bork rewrote monopoly policy in a way that opens the door to exactly the sort of monopolies we now have. But it isn't as if this is some new notion invented because people hate Amazon; it used to be the rule.
Suffice to say this is a recent legal innovation, and many, many people believe ignoring other monopoly effects is myopic.
Amazon is a very big business. It doesn't need your sympathy.
Not saying it's correct by any measure, but it doesn't seem like an unusual practice.