Here, I think, is the key quote, from the jury foreman, Dennis Strader:
The defense argued that the program merely enabled owners of Adobe eBook Reader software to make copies of e-books for personal use. If an owner makes a backup copy of an e-book or transfers it to another device he owns, they argued, that is permitted under the “fair use” concept of copyright law.
Jury foreman Dennis Strader said the argument made a big impact on the jurors, who asked U.S. District Judge Ronald M. Whyte to clarify the “fair use” definition shortly after deliberations began.
“Under the eBook formats, you have no rights at all, and the jury had trouble with that concept,” said Strader.
(From the Associated Press story)