I think you don't understand how game development works. As content is created, produced, and tested, it is included in the current build of the game. Keeping content seperated from the main game could create unforeseen glitches and bugs when it is finally imported into the main game for release - so developers often test or produce content alongside the main game build. That doesn't mean the content is complete or tested - so developers will often hide or restrict access to this content except under their development kits. Basically, just because the content is currently in the game does not mean it is complete or playable, it's probably still being produced and/or tested. Also, a lot of DLC now a days is produced by outside, contracted game companies - freeing Bungie's own employees to work on the normal game.