This is a very common misconception in the United States. It’s how a lot of defenders of antitrust law want antitrust law to work, but it is not how antitrust law does work.
This Supreme Court case explicitly establishes that antitrust laws can be used against companies which obtain a high market share simply by anticipating future demand and responding effectively and efficiently.
b. This 1945 precedent is not the standard that most modern antitrust (post-Bell breakup) cases are held to.
This would make sense as a feature. If you subscribe to the view that competitive pressure is the source of progress, then you never want any company to actually win. Like a donkey chasing a carrot on a stick, you want companies to endlessly run towards market dominance, but never actually get there - because once they do, they stop contributing to progress.
Amazon may or may not legally be a retail monopoly - I do not know the answer. But your question can be rephrased for any monopoly and the answer would be “monopolies should be punished for leveraging their monopoly power in other markets, because that ruins the market for everyone else.”
Free markets and democracies are good at a lot of things, but self preservation is not one of them - therefore you need anti-freedom laws.