Consider what Amazon has done to the industry I know best: university press book publishing.
To highlight just one aspect of its rapacious business practices, Amazon has begun over the past year or so to replace the “Buy” button on book pages with a button that automatically defaults to the cheapest purchase option from any seller, including used book outlets. For a newly published book, those “used” copies nearly always originate as gratis (free) copies distributed to potential reviewers of the book. This means a new copy of the book was never purchased in the first place.
For a scholarly book that will sell only a few hundred copies in total, this means that a significant portion of its sales will generate zero revenue for the publisher or royalties for the author. Consumers will be happy to get a cheap book, but the publisher, which probably operates on a shoe-string, with substantial overhead and extremely tight budgets, will end up getting badly hurt. When this is repeated across dozens or hundreds of books a year, it can turn a lean enterprise that barely manages to break even into a money-losing one. And when such a transformation is replicated across an entire industry, the industry itself begins to lose its commercial viability.