G and Froome are riding the ToB, but they aren't competing for the win. Essentially, having them there is just publicity and a chance for them to pay it back a bit and ride for one of their team-mates.
Given Froome rode the Giro and the Tour, the Vuelta would have been a race too far for him this year, and I think G has been enjoying himself and doing so much yellow jersey stuff he's not ready for something as hard as the Vuelta.