They are certainly not in the public domain. No recent work produced by an individual or company would be in the public domain unless it was specifically released by the copyright owner.

The copyright may not belong to Amazon either. It depends on the review. Amazon doesn't hold the copyright to the Kirkus Reviews or Publisher's Weekly editorial reviews (or similar content) displayed on their product pages. They also do not own the copyright to customer reviews and comments. Amazon has licensed those rights.

In the past, Amazon has said that Associates may reproduce editorial reviews written by Amazon staff, but that's it.

However, if you use Amazon's web services (not sure if it's currently called that) and pull the xml feed, you can display anything you want about the product, including the reviews and customer comments. You just can't copy and paste them into your site.