The other reply to this covers it well. I'll just add one more thing. Some latest versions of the kindle app give you an edition of the ebook that can't so easily be stripped down with calibre. In these cases, a couple solutions exist: first, there's a certain plugin for calibre that does support DRM stripping, which can be installed to the calibre app (i'll try to find it and post here) and secondly (what I myself do), just download and install an old version of the kindle app for your desktop machine and download your purchases to that, they'll appear in their older format which can be DRM stripped via Calibre, and even converted to other formats like epub.

None of this by itself is promotion of piracy as far as I'm concerned, since we're just talking about gaining full control of the things we DID buy and paid for from Amazon, so I refuse to consider it "wrong" despite their tiresome efforts at DRM lock-in.