Reviews are important in the Amazon ecosystem — a clear signal to their search algorithms. Unfortunately, this also means that they are constantly being gamed, and without strict controls on the part of Amazon, quickly become meaningless. Equally unfortunately, Amazon has decreed that one of those controls is a $50 purchasing threshold — ie. if you have not purchased at least $50 worth of products or books from Amazon in the last 12 months from the e-commerce giant, you are not allowed to post. Here is the Amazon page describing the policy.
And the text:
To contribute to Customer features (for example, Customer Reviews, Customer Answers, Idea Lists) or to follow other contributors, you must have spent at least $50 on Amazon.com using a valid credit or debit card in the past 12 months. Promotional discounts don't qualify towards the $50 minimum. You do not need to meet this requirement to read content posted by other contributors or post Customer Questions, or create or modify Profile pages, Shopping Lists, Wish Lists or Registries.