But thats no reason why they couldnt have just included it in the game. Dont forget the Spiderman 3 game on the PS3 could be bought from certain game shops in the US with the Goblin as a playable character on the special edition version, but later made it downloadable for the standard version.
If they were gonna charge for the Joker, then I understand why they are making it downloadable, but as its a freeby, they might as well have included it with the game.