There's the matter of law as well, I can't be certain but it's not out of the question to say Imperial Law forbid duels like that.
Or maybe a law hadn't been yet implemented ? Difficult to say, nobody challenged the High King since the Third Era ?
Well yes, but its a tricky case to take over a land and then implement laws that strictly contradict culture and traditions of the region, accepted for the past however many years or so. Yes, it was an asshole move of Ulfric (seasoned warrior) to challange the young High King (inexperienced) to a fight, when it was pretty clear Ulfric could ascend to power through diplomatic ways as well but as evident by Nord culture, blood equals respect.
That being said, Duels of that nature are more than valid to the Nords, and I would say that Skyrim should be better represented in that regard. However, by using a shout, whether or not he would win with/without, Ulfric delegitimize his cause and that is why I would say it was murder rather than a casualty of a mutually accepted duel. It'd be like if I challenged you to a sword fight, and just as we were about to go at it I pulled out a gun and shot you in the face, only to stab you to death in the end.
I've been looking through UESP wiki and I can't find anything that states that using a shout in a duel is illegal in the eyes of Nords, but from what I can see it is not accepted one bit. If anyone knows anything else then feel free to correct me (I'm looking at you, Dagmar
) but looking at the situation as a Nord, honor is the first priority and what Ulfric did was far from honorable.