When you read it your also copying it into cache. We could argue the technicalities but really it's about copyright law not the colour of bits. Or if you prefer we can just say it's legal to copy the ebook to a new owner if you remove your own copies (which is one process the above poster refereed to by writing "bar some caveats").

Well, e-book vendors ought to either use a shared format or provide a mechanism for transferring ownership. They are unlikely to do so unless a right to resell is established. I am pessimistic about the likelihood, but it would be a very good thing for the world.