On the Fact that People Are Putting Pressure on Amazon.com to Stop Selling Books by Holocaust Revisionists

What are they afraid of? And how is it that we can still debate the French Revolution, the American Civil War, the Theory of Evolution, and Communism but we can't investigate what is alleged as the greatest mass murder in human history? I mean, what happened to, "extraordinary claims require extraordinary evidence" (and, no, ludicrous testimonials by proven liars such as Irene Zisblatt and Elie Wiesel just won't cut it - sorry)?