Destiny appears to be free on the Microsoft Store, but Bungie has clarified that the game is not free.
Instead, the download only features the trial of the game. The statement came via Bungie’s website and says:
The original Destiny adventure may appear to be free on some digital console marketplaces.
While players may download the Destiny base game, they will only be granted access to the Destiny Trial Mode for free. If players wish to progress through the original Destiny adventure via digital download, they will be required to purchase Destiny – The Collection to gain access to this content.
Destiny – The Collection is set to be released alongside the game’s next expansion, Destiny: Rise of Iron. The Collection features all the game’s DLC, including Rise of Iron.
Destiny: Rise of Iron and Destiny: The Collection launch on September 20th.