Unless it's specifically stated, the reviews would not be "in the public domain." The copyright is most likely held by the person who wrote the review.
You might find this thread interesting:
As I understand it, this comes down to the difference between licensing/allowing someone to use your work and transferring the copyright to them. If you read the TOS for most forums, etc., you'll probably find some form of licensing or permission language rather than transferring of copyright, because transfer of copyright is legally more difficult to do.
By statute, in the U.S. at least, an actual copyright transfer has to be done in writing with a "real" signature (I'm not an attorney, but I do deal with this all the time in my "day job"). However, by accepting your site's TOS or something similar the copyright holder can give you permission to use their material. You just don't own it which, in part, means you can't give permission to anyone else to use it - they'd have to go directly to the copyright holder.
An example I've run into by being an amazon.com associate is their customer reviews. Amazon.com allows me to post their editorial reviews on my site, and I often do. They can give this permission because those reviews actually belong to them - they hold the copyright. OTOH, the TOS for the associate program state that I cannot post customer reviews on my site. Amazon.com would have no legal right to give me permission to do that because they don't own the copyright - they've simply been given permission by the review writer (who holds the copyright) to use the review on their own site. They can't pass that permission on to me.
I think there's a different application of this for associates who use some of Amazon.com's web services, because the material stays on Amazon.com's server so isn't really being re-published, but I'm not sure.