if you live in the US, the DMCA states it is illegal to produce, distribute or use software designed to circumvent technical copy-restriction (DRM) methods. this is regardless if they are your "own" files or someone else's. under most TOS you don't "own" the applications or ebooks, you simply license the right to use or read those applications or ebooks.
so if you use the various scripts out there to strip the DRM from .mobi or .epub files that you have in your possession you are breaking the law. fair use rights allow you to back up the copy protected file, not strip the DRM and back that up.