Well as a matter of law I would favour Hitler's "Mein Kampf" being freely available, and not forbidden. But that I would allow it doesn't mean it can't cause any damage IMO
First of all amazon.com has issues of Mein Kampf so what you mean by forbidden I am not sure.
Perhaps he's speaking of outside the U.S. There are countries in which it is not legal to sell Mein Kampf, e.g. the Netherlands (
http://www.rnw.nl/amsterdamforum/html/031128hitler.html) or it's difficult to legally sell it, e.g. Germany (
Page not found | WIRED).