The DRM used on ebooks is pretty weak and easily removed. So in practice, there's no real difference. The trouble is that doing so is a violation of the DMCA,

The DRM used on ebooks is pretty weak and easily removed. So in practice, there's no real difference. The trouble is that doing so is a violation of the DMCA, which can result in criminal charges. So the simple act of keeping a DRM-free copy (to prevent Amazon from yanking it off your Kindle), or lending it to a friend, etc. opens up all kinds of potential problems in theory.