Most of us live in a democracy, laws aren’t black and white and can be changed on a whim with little more than a single election cycle, or interpreted by courts in any arbitrary fashion. Sure, a large company can push unfair practices to a point, and can throw lawyers at it to a point but I think some large companies fatally underestimate the danger of flouting their position; the US government or a pissed off Judge could shut down Amazon’s entire operation in a day if there was the political will or some scandal came about. So it is absolutely in their self-interest to make sure their practices are “fair”.
So as is often the case, the legal system is quite conservative whereas people wildly claim that whatever they don't like at the moment -- whatever seems "unfair" should be illegal, and fortunately there are usually enough roadblocks to prevent that from happening.
> the US government or a pissed off Judge could shut down Amazon’s entire operation in a day if there was the political will or some scandal came about.
Nope.
Even if there are pissed off or crusading judges -- of which there are plenty -- those decisions will get appealed and overturned via the appeals process and it definately will not take one day, these cases can take a decade as they wind their way to the supreme court where they are finally turned into nothingburgers most of the time, to the constant fury of those who insist that their ever-changing moods should be encoded in law, or that a pissed off judge should be able to carve apart a company. There is a reason this has never happened in the history of the U.S. Sometimes monopolies are broken up, but you can usually count on one hand how many times that will happen in your lifetime -- I think ATT was the last time that happened, before most of the people on this site were born. And it hasn't happened since. The judicial system just doesn't have this power and laws are very slow to change, requiring much more than just a shift in public opinion.
Pretty much in agreement with everything else, but had to note that people viewing the world that way would not have much experience with real HR departments. Maybe there is another reason for the misconception.
But that's not how it works, you have to actually violate some law on the books, and I'm not sure what laws against monopsony we have -- for example, Apple is ruthless in driving down prices (for itself) due to its market power, and if it forces vendors to agree to never charge Apple more than what they charge another customer, this would not be illegal, even though the end result might be vendors raising prices for everyone except Apple, and thus seeming "unfair" to everyone else forced to compete with Apple. Demanding the best possible deal is not unfair competition.