Is it actually ethically acceptable to control distribution of something that naturally shares itself?
Before computers, it actually required some energy to copy the content of a book.
With computers now, the action of reading an ebook will actually copy it from the hard drive to the ram. If your book is on the cloud, there's even more copying going on.
It actually takes more efforts to erase temporary copies (ex: from local cache)!
Digital copying is not the same as physical copying.