Why do people write Amazon book reviews?

As I’ve given talks and written the past couple of years about the motivation mysteries surrounding user-contributed content sites, I generally mention Amazon book reviews as a prominent example. It is not uncommon for over 100 people to review a popular book. And the top 10 reviewers (as of today) have each written more than 1600 reviews (leader Harriet Klausner is about to pass 17,500!).
Why? Not only is that a lot of time (allegedly) reading, but it’s a lot of time writing…for the economic benefit of Amazon. What do reviewers get out of it?
One explanation for open source software contributions is that new programmers get professional experience on a team software engineering project, and their contributions are publicly documented so they can show them to potential employers. That might explain some reviewers on Amazon: they can show their reviews to an employer, and users rate them so they can show their scores too. But how many jobs are out there for book reviewers (and what about those with massive output who remain “amateur”)?
Slate published Garth Hallberg’s article in January 2008 (yes, I’m a bit behind in posting things to this blog!) that suggests the amateur reviewers may be motivated the old-fashioned way: through extrinsic, direct benefits. For example, apparently publishers send free copies of their books to prolific reviewers, so people who do want to read a lot get a lot of in-kind compensation. Grady Harp (#6) said he is “innundated”. Amazon has extended this form of compensation by creating its Vine program, in which it selects successful reviewers and gives them free products from across its line of goods (electronics, appliances, etc.), as long as they write reviews (Amazon claims they do not influence opinions or modify or edit reviews).
(Thanks to Rick Wash for pointing me to the Hallberg article.)