I still don't get why they had to pin the blame on Batman, though. Keep Dent as the heroic symbol, pin the deaths on the Joker. If they're going to lie, why make it a stupid lie?
Enough people would know that the Joker was terrorizing the ferries and captured by Batman while Gordon's son got kidnapped and Dent killed. It'd be hard to "gag" that many people for such a lie to work.
But why pin Dent's death on any
real person? Why not say Dent was killed/kidnapped along with Gordon's family by a factious person who remains at large?