An Elder Council member is also high up in politics. Same situation, same type of punishment.
Ulfric, in the eyes of the Empire doesn't have 'provincial monarch' status. He even has wanted posters for his arrest like a common criminal.
"You are traitors and will die traitors' deaths."
Trials rarely happen however, the 'Guilty until proven innocent' has been around since the First Era apparently. They are next to non-existent to the eyes of citizens, besides one about an Emperor (Who wasn't facing High Treason). Two about High Treason (One was Ulfric) both received the same thing.
Ralof is saying they didn't dare give Ulfric a fair trial, then mentions 'Treason' with skepticism like the Empire branded him as such to execute him without a full hearing.
"Ulfric Stormcloak! You are guilty of insurrection, murder of Imperial citizens, the assassination of King Torygg, and high treason against the Empire."
He's already guilty, Tullius was his trial at Helgen and Windhelm. Being summarily tried and executed is the fate that awaits if he's caught.
Majority in TES don't get fair trials, so if you were a citizen you would expect the same. Guilty until proven innocent has been around for so long, even during the Second Era it's mentioned in a source trials non-existent for the smugglers.
In the Third Era, a citizen was arrested indefinitely for doing the Night Mother Ritual. Ulfric wasn't given a trial when he was arrested in the Markarth Incident. There are no mentions to any prisoners in Skyrim getting trials either.
The only sources mentioning trials, is Ralof who believes Ulfric accused of treason is a lie and an deposed Emperor who was going to a trial.
He gets gagged, they're unlikely to remove it to let him speak in the presence of the Emperor. Especially when rumors that he killed Torygg with his voice, are floating around. Even if that is what he wanted, he wasn't going to ever get it.