The reason, they say, is because they sell less, thinking that people would like to copy the music to their portable players etc.
Sony must be one of the companies that mostly sits in 2 chairs, as both a hardware and software-producer.
Must feel sort of stupid to sell various recordable medias, at the same time as they copy-protect all their music which makes their recordable equipment halfway useless.
Which I think is a major reason why they do it.
Copy Protection has extremely limited use. It's not as if copy protected songs wouldn't end up being shared anyway. That way, all it does is making life harder for people that actually do go out and buy the music. The CDs might not work, they can't make a copy to their portable player etc.
I guess, it was tough for Sony to sell various portable players and keep a straight face doing it.
A solution, in that case, would've been to make some kind of coding inside the players that allowed them to rip a compressed copy of Sony CDs etc, but I doubt they wanted to take that path, with all legality problems etc..