That seems rather odd. Ive had more than 6 Kindles and devices, way more, and if that rule was true then I would not have been able to DLed purchaed books to all of them. Over time I purchased many new Kindles/devices and at some point I passed 6. Many of those Kindles/devices are now sold and no longer registered to me. I've never encountered that issue, so perhaps there is something else going on which you need to contact Amazon about.