Well, firstly because the explanation as laid out here and here seems fairly plausible to me.
But mainly because I just don’t think that Amazon are that stupid. They sell an enormous amount of material which is liable to offend *somebody*: they sell books about being a gay parent and books about curing homosexuality through God. They sell books by young-earth creationists and books by angry atheists. They sell box sets of hardcore porn DVDs. They sell books by holocaust deniers.
The last thing they want is to be seen as endorsing any of those things. It is completely in their interest to be perceived as a non-judgemental buyer and seller of goods; the people who will sell anything, as long as its legal. The moment they are seen as exercising some kind of editorial control on the basis of a moral or political agenda, they lay themselves open to having to defend every product they sell.
They cannot be seen to be taking sides. And I think they know that.