Amazon puts "Used" for anything that can't be sold as new. Because of the boxes being damaged they can't sell them as "New" because people wouldn't want to pay "New" prices for a dented box. Amazon doesn't have a New or Used condition specifically for damaged boxes.
I've bought around a dozen things from Amazon warehouse. I have never received anything that was actually used, it's always been a damaged box. I always get the ones that say, "...but packaging has damage", I have not tried to buy one that says "pristine" packaging. That seems odd too me and sounds like a return.