I think the theory is FB gets to keep data on who liked/followed/interacted with the page/content, which is still valuable data to mine for profiling the users.

I think the theory is FB gets to keep data on who liked/followed/interacted with the page/content, which is still valuable data to mine for profiling the users. A true removal of an account should result in all interactions with the account and its content being removed as well, which I really doubt is the case.