This is more complicated than it sounds. Amazon is continually trying to stay ahead of de-DRM software. In many cases you have to use older versions of the Kindle software to download, or use de-DRM software external to Calibre to do the DRM removal. I download recent purchases from Amazon directly to my Mac and the de-DRM addins in Calibre didn't do a thing to them. I haven't yet tried standalone de-DRM scripts on them.

Not once in my many years of pirating books have I come across a book I need to remove DRM from.