Well, if the owner of the forum is receiving request e.g. to delete accounts or to disclose what data is recorded about someone, why not just comply with the request? What's the big deal?
If I ran a forum for a number of years, and a person decided to close their account, that'd be fine. But if they then said that I need to remove _all of their posts_, that's really shitty. It would destroy the usefulness of a forum.
Does that also mean then that quoted content counts? What if another user merely copies/pastes text someone said into their own reply (like you have to do on HN)?