Attributor Protected, means that the company digitally watermarks the eBook. Thus if the eBook is later found on a Torrent or MobileRead, Mobilism etc the digital watermark can identify who, when & where the book was purchased, at least that's the theory.
It's an alternative to implementing DRM as copy protection. The company scours the web looking for its 'protected material', & digital watermark....
http://www.digimarc.com/products/guardian
http://en.wikipedia.org/wiki/Digimarc