What Facebook essentially did was grant themselves the right to all user-uploaded content for, well, forever. It no longer mattered if you removed anything from the site, because it would remain in Facebook's archives, giving the site free reign to use the content for as long as it likes.
To justify the decision, Facebook compared the policy change to that of sending an email to a friend. Even if you delete the sent email from your sent box, a copy still remains in the recipient's inbox, so according to Facebook, it was okay for the site to keep and use your content.
Here's one for Facebook: The squeeky wheel gets the grease.