Freedom of speech was included as a protection by the founding fathers to preserve freedom, not promote societal decay. Freedom is preserved when those out of power are able to project their voice past those who would be inclined to stifle it. The book in question is about promoting the exploitation of the innocent by powerful adults.
Nor is Amazon censoring by refusing to retail such trash. It would merely be exercising its civic conscience by deciding not to trade in the promotion of exploitation of innocent boys by men of dubious moral character. The book would still be available, Amazon just wouldn't assist in its distribution. Censorship is when government decides what can be printed. That is not the issue here.