What do you think will happen to korkron after the siege? These people have been fighting for us for many years in outland, northrend, azeroth. Trying to win wars, save lives. Now to show that garrosh is bad, all of them have been demonized. Will they be disbanded, killed, reformed ?
Here's the thing. We can't see it in-game, but I would bet money that these 'Kor'kron' are not the same Kor'Kron that Thrall commanded. At least not most of them. The Kor'Kron were the elite, and the elite would be very loyal to the beliefs of the warchief. As for these Kor'Kron..they'll probably die in the service of their current Warchief and be happy to do so. Lok'tar Ogar, and all that.
As Stelaris said, I think these Kor'kron aren't loyal to Thrall's Horde - but Garrosh's. They were probably brutes hand-picked for ruthlessness! Most of the mobs we'll fight in 5.4 will most likely be sha-tainted Kor'kron.