Copyright infringement is about publishing, not consuming. If I copy a friend's legally-purchased .epub that's like borrowing their legally-purchased printed book. Same as if I take a book from a Little Library in my neighborhood, or borrow a paper book from the city library. Why is the transfer of digital content more restricted? A print publisher can't prevent a library from lending their book as many times as they want, so why do we give digital publishers that ability?
Quoting Wikipedia: "Copyright infringement (at times referred to as piracy) is the use of works protected by copyright without permission for a usage where such permission is required, thereby infringing certain exclusive rights granted to the copyright holder, such as the right to reproduce, distribute, display or perform the protected work, or to make derivative works." - https://en.wikipedia.org/wiki/Copyright_infringement
Copying a friend's legally-purchased epub is copyright infringement the same way that making a photocopy of a physical book is copyright infringement.
It is not the same taking a book from a library due to the "first-sale doctrine", which Wikipedia describes as "an American legal concept that limits the rights of an intellectual property owner to control resale of products embodying its intellectual property. The doctrine enables the distribution chain of copyrighted products, library lending, giving, video rentals and secondary markets for copyrighted works (for example, enabling individuals to sell their legally purchased books or CDs to others)." - https://en.wikipedia.org/wiki/First-sale_doctrine
> so why do we give digital publishers that ability
Because people agreed to special licensing terms that restricted what they could do, in favor of convenience.
So if a friend has their one copy of a legally-purchased epub on a USB drive, they can give that USB drive to me legally, right? and I can read the book and pass the drive onto someone else?
If it came from a Kindle, the license agreement your friend agreed to as a condition of using the Kindle Store says "Unless specifically indicated otherwise, you may not sell, rent, lease, distribute, broadcast, sublicense, or otherwise assign any rights to the Kindle Content or any portion of it to any third party, and you may not remove or modify any proprietary notices or labels on the Kindle Content." https://www.amazon.com/gp/help/customer/display.html%3FnodeI...
Is the content protected by DRM? In which case you can be subject to the Digital Millennium Copyright Act, which "criminalizes the act of circumventing an access control, whether or not there is actual infringement of copyright itself." (Quoting https://en.wikipedia.org/wiki/Digital_Millennium_Copyright_A... ).
If your friend bought the epub on a USB stick from a store, and there is no DRM, and no so-called "shrink-license" which forbids transfer (see https://en.wikipedia.org/wiki/First-sale_doctrine#Ownership_... for how that plays out with software) then you are fine, in my decidedly non-lawyer opinion.
Note how that last Wikipedia link comments "if someone buys MP3 songs from Amazon.com, the MP3 files are merely licensed to them and hence they may not be able to resell those MP3 files. However, MP3 songs bought through iTunes Store may be characterized as "sales" because of Apple's language in its EULA and hence they may be resellable, if other requirements of first sale doctrine are met."
Which is why you need to know what your friend means by "legally-purchased."
The original US copyright law was 14 years with a 14-year extension if the author was still alive. Certainly financial interest (Disney) is to blame for that incrementally becoming 70 years after the death of the author.
