I guess, think of an ebook, I could buy a Steven King e-novel, I own the novel but King still owns the copyright. I can't make 100,000 copies and give them away to my neighbors.... if I had 100,000 neighbors! LOL!
Actually, that analogy would work for the game, not custom content. Obviously, one can't copy the game and give it away to friends. A book's copyright and the game's copyright is explicit about that.
However, if one so chose (and there was any purpose in doing so), one could take a quote out of their book, change the font, and insert a piece of paper with that replacement quote into the book that the friend bought.
Yeah, my analogy sucks about as bad as the one I refuted, but whatever. :x