The issue isn't technical or legal. It' just that Bungie is working on releasing the game on four platforms at once. Xbox One, PS4, PS3, and 360. They just don't have the people to put towards a PC release right now. Bungie has stated that they are interested in bringing the experience to PC but they want to make sure that every version of the game is polished properly for each platform it will be available on. They don't want to cut corners. I'm sure once the console release is done they will have a team working on a PC version.