If they had let Froome go, they would be 1-2 at the end of tonight and the win would be guaranteed. As it stands, there is still a slim chance it could all go wrong, ala the Vuelta (although highly uinlikely it must be said)
It appears obvious that Froome was taking Nibali with him and away from Wiggins. Sky couldn't allow that. I think if Froome had made the break on his own Sky would have let him go.
Why don't you get it. Froome is the most naturally talented cyclist right now, and this is the greatest performance by a domestique in the history of the Tour De france.