Mike L,
As mentioned in this forum, there is a huge difference between setting up an Amazon account which anyone can do without a Kindle, and registering the device itself. That is to say, making your Kindle synch with the details of your Amazon account via Whisphernet
Further, it is perfectly possible to use a non-registered Kindle to read and buy books bought via your Amazon account. That was never in question.
Please check the Settings page of your Kindle to check who it is registered to. Incidentally, my Setting page looks like this:
