Originally Posted by pdurrant
So long as the text is wrapped in DRM, the DMCA protects the text, even if it's public domain text. Not directly, but because the tools needed to remove the DRM are illegal.
By default, ebooks from Amazon's DTP are DRM-free. So the device "limit" could be entirely notional, even though it is still enforced on Amazon's servers for downloads. There is perhaps disagreement about whether it is legal to strip DRM from an ebook for personal use (and, as you say, developing the removal tools is illegal), but there is little doubt that copying a DRM-free file to a 6th or 7th device for personal use is fair use in the US.