Flint firestorm
The leading man, who else?
Even if (I say if because I've never seen it said in tamrelic lore that they can) they have that right it's not what skyrim wants. The majority of the jarls want to stay with the empire.balgruuf chose to support the imperials. therefore, 'betrayal' is not a valid response.
the banning of talos, a corrupt, immoral and weak empire, and the treaty is more than enough to justify withdrawal from the empire.
in fact, skyrim (nords/jarls,etc.) has the right to withdraw and demand autonomy at any time.
'detrimental to tamriel' is irrelevant. 'too vulnerable' is irrelevant. even if it was known that all the inhabitants of skyrim would be utterly destroyed by the thalmor in a war doesn't change the fact that the reasons for skyrim's decision to want autonomy are valid.
listening to ulfric's dialogues has proven to me that his actions are righteous and just.