If a user posts a review on Amazon, does (s)he have copyright on that review? Is there anything in the terms and conditions where (s)he transfers his/her ownership of the IP to Amazon? If not, how does Amazon protect its reviews from being used on other sites?
The author retains copyright.
Amazon does not claim to control or own customer reviews. Instead they claim a right of "sublicense" where they are allowed to republish the review forever. They specifically use the term "nonexclusive". They claim no IP rights over your review, just the ability to display it (and parts of it) in any form on any media.
unless we indicate otherwise, you grant Amazon a nonexclusive, royalty-free, perpetual, irrevocable, and fully sublicensable right to use, reproduce, modify, adapt, publish, perform, translate, create derivative works from, distribute, and display such content throughout the world in any media.
At the time of writing, the link to their Conditions of Use is here: