Amazon doesn't write and publish books and there's no technical issue in others distributing them so - no monopoly other than the quasi-monopoly established by people choosing to use Amazon as against other book sellers. Similar arguments apply in the transient opinion distribution industry currently dominated by Facebook and Twitter. These company's powers are entirely in the hands of their users who have a choice and should remember that today's market will likely look quite different within a few decades if not sooner.

If you want a book-seller who doesn’t censor your buying options, don’t go to Amazon. Plenty of other sources.

Quasi-monopoly is still a monopoly.