That isn't how Nordic law works.
 
Killing Torygg is in absolutely no way treason, especially because he accepted the duel knowing Ulfric was a better warrior and knowing Ulfric was a Tongue.
 
Jarls can challenge the High King for the throne. That's what happened. High King Torygg betrayed his people to the Thalmor and Ulfric put him down in a way to show that the new High King would be a powerful defender of Skyrim - by using the Voice, like the Tongues of old.
 
If I remember correctly, the moot has only been called once after a High King was defeated? At least, that's what I read from another poster a long while back. Seems like Moots didn't become common place until the Empire started bribing jarls with imperial coin.