> I like a conspiracy theory as much as the next guy but couldn't the book just be... unavailable?

Its unavailable in the sense that Amazon doesn't make it available -- so it is, literally, unavailable from Amazon. Strictly speaking, this is not inaccurate, it just doesn't mean what people who trust Amazon's comprehensiveness are likely to think it means, as its not unavailable from the publisher, which is why its also not unavailable through other retailers.