Because all content you post on your Facebook page belongs to them, to do with as they please; and it stays that way, even after you have your account canceled. Or, as their Terms Of Use put it:

You hereby grant Facebook an irrevocable, perpetual, non-exclusive, transferable, fully paid, worldwide license (with the right to sublicense) to (a) use, copy, publish, stream, store, retain*, publicly perform or display, transmit, scan, reformat, modify, edit, frame, translate, excerpt, adapt, create derivative works and distribute (through multiple tiers), any User Content you (i) Post on or in connection with the Facebook Service or the promotion thereof subject only to your privacy settings or (ii) enable a user to Post, including by offering a Share Link on your website and (b) to use your name, likeness and image for any purpose, including commercial or advertising, each of (a) and (b) on or in connection with the Facebook Service or the promotion thereof.

So they own anything you post and have posted, and they can use it in any way they see fit. They can use your pictures for advertising. I'm not even sure what the implications would be if an artist puts his song lyrics on Facebook for the fans to enjoy. But surely no one would do that...
or would they?
If you want to know more, see this article.
And if you want to know even more, read Facebook's response to that here. Honestly I'm underwhelmed, but see for yourself.

And is LJ any better? Yes, in this regard it is.  Because in LJ's Terms Of Service, I found this:

All Content posted to LiveJournal in any way, is the responsibility and property of the author.
and:
LiveJournal claims no ownership or control over any Content posted by its users. The author retains all patent, trademark, and copyright to all Content posted within available fields.

Now that sounds a lot more reasonable to me. What say you?

*) Yes, after you've deleted it off your page, it's still theirs.

Update as of February 18:
Seems they have caught enough flak to make them reconsider.
See this blog entry.