I think we have to draw the line with abusing children. I'm not for censorship. . .but I do believe child pornography is a crime, and I think a book giving pointers on how to engage in sex with children crosses that line.
I would hope that the powers that be would investigate anybody that buys that book, just as if they browsed a child pornography site. I can't imagine that anybody would actually publish that book, let alone anybody would feel emboldened enough to buy it.
Amazon is free to do whatever they want. And I am free to not do business with them, which I won't.