This is censorship, and it's a slippery slope.

IMHO the right way to go about this, would be for amazon to attach a letter to the cover of the book saying something like: "We're sorry you bought this book, but we do believe in your right to choose what you do with your time and money, so we have not removed this book from our store. FYI, according to (insert number of) scientists and XYZ peered reviewed papers, the vast majority of the content of this book has been certified to be false and you should seriously question the author's recommendations before acting on them, as they may pose health risks. If you require further information, here's some websites & books that provide what is widely considered to be factual information on the matter".