If they were to allow anyone to put whatever up on their site, it will cost Sony more money in storage and bandwidth. So, there is a hard cost with opening the gates. As a reader, I'd like to see them do that, of course. Is it good business sense? I doubt it.
Also: So many companies like Sony are doing whatever they can to protect their distribution (music, video books) channels that allowing others to control that aspect probably feels pretty icky to them. And, yes, 'icky' is a bonafide, real economic indicator!
