Possibly yes. When a publisher choses to sell through Amazon I assume that includes some form of legal agreement between the two parties. This idea that "They're a private company, they can do what they want" is wrong if they have some kind of agreement to sell the book.
The fact that we've been letting the tech giants get away with putting vague language into their terms of service and using that as a justification for arbitrarily enforcing or even changing the rules is bullshit. These catch-all provisions that let companies "do whatever they want" would never have stood up in court in the past. They are undermining our ability to conduct business and are being abused to stifle competition.