I recently was given a few old PC's, I was amazed to find there was still personal info on the hard drives. I always wipe the drives with Dban when I get them like this. I think this is one of those things that we really need to teach people about destruction of data before releasing it into the wild. (Admittedly I have a lot to still learn about it my self). We would never just throw your old tax papers in the trash before shredding them. That is what we are doing when we put HDDs out in the world without at least a deleting the information or formatting the drive. Neither of these truly destroys the data, but; for most people it would seem that way. Ultimatley any data can be retrieved from these disk, it is just how far would some one go to retrieve it. To truly destroy the data you could physicalley destroy the media.
I believe most people are honest and will at least reformat the drive or delete the old files, It unfortunately only takes one jerk.
Here are a couple videos of destroying HDD's