Now, magicking dead actors back to life using skillful editing and digital effects isn't a new thing -- there's John Candy in Wagons East, Brandon Lee in The Crow, Oliver Reed in Gladiator, and Paul Walker in Furious 7, to name a few. Except in all of those examples, the actor in question died while the movie was still in production, having already completed some or most of their scenes. Cushing had been dead for two decades when Rogue One went into production. In other words, Disney executives got together and planned to have a long-dead actor in their new Star Wars film. The potential for fan nostalgia outweighed the need to respect the man's life by acknowledging that it had ended. And the very last shot of Rogue One is of a CGI Carrie Fisher, reprising her role as 19-year-old Princess Leia.
Now, Lucasfilm (and Disney) own the likeness of Carrie Fisher as the character Princess Leia. Just as they own the likeness of Peter Cushing as G-Moff Tarks. They don't call everyone who has ever been in Star Wars to ask their permission every time they make a new action figure or video game.
If they did, "Harrison Ford punches Lucasfilm intern" would be a monthly headline.