The AP news wire works differently than other sources. AP is written to be used by news agencies and to be reprinted in papers, used in broadcast journalism and now used on the net. I may be wrong on this (I'm digging deep in my memory from college which was almost 18 years ago), but I think AP may be a paid service that news agencies use, so they are, in effect buying the rights to use the info in their articles. Someone correct me if I'm wrong.
A fellow writer writing text for anything, whether that be a town's web page, a magazine, the internet, etc., is not getting any cut when someone uses their work in an article. But, someone else is geting paid for the work that came out of the first writer's efforts. That's not ethical.
No, I don't think it is plagiarism when you list the ingredients in your product reviews. That in itself is the source, so you are fully citing your source, plus it's just a list of ingredients, it's not intellecutual property. Now, if you were to take the product description from the advertising and reprint it word for word without quoting and citing it, that would be plagiarism.
ETA - I just looked at the AP site and they do have a link on the bottom of the page to buy their news, photos and videos. I bet they are plagairized frequently by people who lift things from their site without buying it or by people who lift the material from their local newspaper. I think this was a problem on AC actually and that is why they got tough with news.