This is about ownership of your posted items - including videos, pictures, and words... amongst other things.
Not so much about privacy -- it's about claiming ownership of your files.
Consider Facebook is really just a web host. You upload your files to this webhost... except this webhost (facebook) tells you they own everything you upload there... forever. Even if you delete your webhost account, and delete all the files on there, they own it all forever.
Would this be acceptable if it was your webapp being hosted... and the webhost said from now on, 'all your files are belong to us!!!! 4Evar!!!'?
Why do people expect privacy in a website in which one's goal is to display their vanity to everyone?