Now before you go ape sh!t on me, i have a few points.
Why can't the Hokages fight?
Believe it, if Orochimaru wanted to use the Hokages, they would be on the battlefield by now. He only wants them to talk to sasuke and therefore they have no power.
Juding from them just talking, it seems they have no intent to break free.
The only way they would break free is if Minato learned of Naruto fighting Madara and Obito