"Throwing a tantrum" might be a little much...
You got to see if from his point of view. There is a nerve bewteen your jaw and your skull and when you get hit on the chin like that it pinches this nerve which cause you to black out and your knees to buckle. And during this time you can't defend yourself, thus the ref stoppage. A 100% correct and timely stoppage in my opinion.
But to Kos, it was: "Oh man, I'm picking apart this BJJ guy, after this fight, I'll take on another WW and get my shot at GSP again. Me and Fitch did the better against GSP than anyone else, I think I can take him this time and then I'll be able to... Why the hell did you stop the fight, ref??"
After you recover from this little black out you are basically 100% back to normal, esp in this case, as it wasnt a terribly hard uppercut, just one that was right on the button. So after the ref stopped it Kos was 100% fine, thus he was a little upset. But if the ref didnt stop it then, Thiago wouldve been on him and landing 3 or 4 more blows and Kos wouldve been asleep for at least a couple minutes, and he would not have been 100% fine upon waking up.
But his reaction isnt anything too surprising, as has been said before; he trained for months, he was utterly winning the fight and then he gets caught, the ref stops it, and immediately after the stoppage Kos is 100% ready to continue fighting, I'd be pissed too.