Speaking in a recent interview, CD Projekt RED’s Konrad Tomaszkiewicz explains that the studio don't charge for DLC because they see it as an post-release service to their fans. He also talks of the decline of the expansion pack and how DLC's too small to fill the same purpose.
CD Projeckt RED have a history of updated their games with extensive additions for no monies.: both the Witcher games received new chapters, graphical updates, and engine optimisations. “We consider DLCs as a normal post sale service, which shouldn’t be priced,” said Tomaszkiewicz, in an interview with VG24/7, “Back when retail games were dominant, we had expansion packs. These were really large chunks of content, which were worth their price.” In his opinion, it's not that a developer shouldn't ever charge for a DLC but “if today’s DLCs offered the same amount of content, they would be worth paying for.”
He also suggested that the studio plan to continue offering free expansions as “a way of saying 'thank you'” to those who bough their game.