There have also been cases where a customer violated Amazon's terms of service and lost access to all of their Kindle e-books. Amazon has all the power in this relationship. They can and do change the rules on us lowly peasants from time to time.
Note, there are indeed ways to download your books and import them into something like Calibre (and remove the DRM from the books). If you do some web searches (and/or search YouTube) you can probably figure it out.
I looked in to the whole DRM removal thing. From what I could tell, everything was majorly out of date, required a really old version of Calibre, and didn't work with newer books.
Edit: So, this is out of date info. There's a fork and it works with a fairly recent version of the PC app. Basically no fuss.
The DeDrm plugin and the most recent Calibre worked for me just yesterday on a brand new book. Something that's easy to miss is that you need to put in the serial number of your kindle device and make sure you download the e-book for that same device. Otherwise the plugin won't be able to decrypt it.
Yep, looked it up again today and some proper information has been posted publicly in the interim since I last tried. I was able to strip the drm from a handful of my books today using it and an older version of the Kindle PC app.
Calibre can be recent - the plug-in was abandoned but forked, but an old physical Kindle is beneficiary. However some books in the store are no longer available for those lately.