THe issue is that digital content is much different than physical content.
If I buy a paper book and read it and sell it to a used book store, I no longer have a copy. And when someone buys it used, the publisher lost one sale.
If I buy an e-book, I could strip the DRM, keep my copy and give it to multiple friends, put it on torrents site where 100s or thousands may download it and give it to their friends, put it on different torrent sites etc. All the while I still have my copy.
So publishers have a lot more to lose from e-books being pirated than they do used book stores. Each physical copy of a book sold and the traded/resold etc. can only be in one persons hand's at a time. One copy of an e-book can have DRM stripped and then spread to an unlimited number of people.
The shift to digital media is a terrible frightening thing for publishers/authors, record labels/musicians, video game developers, movie studies etc. as the ease of people getting copies of your products without paying for them is vastly greater than it was in the past.