From my understanding, it's a common thing nowadays for when actors sign on to do the part, they sign away their likeness for that particular movie/franchise. It's not like, for instance, Aliens where toy companies had to negotiate with the actors for their likeness rights because their wasn't originally a merchandising deal. Instead, the toy company acquires the Dark Knight license, allowing them to create any character from the film. They don't have to consult Bale or Ledger's estate to make a Batman or Joker in this kind of deal.