You still have to pay
Okay, let’s unpack the headline. Batman: Arkham Knight is being published by Warner Bros. Interactive. WB is a consumer-gouging publisher, so Batman: Arkham Knight has a $40 season pass with Batgirl, $100 premium edition, and launch DLC exclusive to certain retailers.
The forum community admin just cleared up the retail-exclusive DLC release plan. That content will be timed exclusives and available to anyone who buys a season pass or the premium edition several months from now, after that contract expires.
“Throughout the 6 months of additional Batman: Arkham Knight content, Premium and Season Pass owners will receive all in-game content that is offered through various retail pre-order incentives. This includes the Harley Quinn and Red Hood Story Packs, as well as any Batmobile or Booster Pack that was available as early bonuses for pre-order of the game. For those who did not receive the content with their game, these items will become available when their respective exclusivity windows expire in August/September.
“The tl;dr version of this is: Premium Edition & Season Pass owners will receive all retail exclusive DLC after their respective early access periods expire.”
The PS4-exclusive Scarecrow DLC will make it everywhere, too, around the same time.
Understanding the Batman Arkham Knight Release Model [Batman Arkham Knight Forum via PC Gamer]