Not only might you not have purchased from Amazon and instead bought it some other way, but

* Most early reviews are via ARCs (Advance Reading Copies) which you get through netgalley and similar sites, or directly from the author/publisher

* It is quite likely that you're reading something (legally) without having bought it because libraries exist

* Just because you bought it, doesn't mean you read it, and you might be tempted to leave a one-star review based on the fact that it shipped slowly and arrived late