Amazon does not save the place of publication and therefore citations might be incomplete, that is right. But I would say that is still better than to save "wrong" data or slighly different data. Let me give an example: http://www.amazon.de/dp/B002RI9ZY0/
If you are using the WorldCat look-up, then you will receive the data from a different version/edition of this title with slightly different data: title is different, publisher is different (and HarperCollins is seated in NewYork not in Boston as Houghton Mifflin is), publication date is different. There are also other examples with written books only. There might be an edition with larger font size and the publisher decides to "reuse" the ISBN. This could lead to problems when citing a specific page of this book (look up with the ISBN in WorldCat).