If you pay a great attention to the lore, you will realize they are a bunch of hypocrites.
1. Ulfric Stormcloak drove Forsworn out of the Reach. Well, for someone who fights for his native land, he doesn't really have much respect for the other natives and their land. (Forsworn are the natives of the Reach)
2. They blame the Empire for banning the worship of Talos. Well, the Empire was almost destroyed by the Thalmor. The treaty that banned the Talos worship was most likely the only choice the Empire had.
Stormcloaks: ORLY? Then it's the empire's fault for losing the war !
Well, you nords fought in the war too as Legionnaires and lost... including Ulfric himself... so don't go blaming the Legion for losing the war.
3. True sons and daughters of Skyrim? What about the Falmer, when they used to be the civilized Snow-elves. Nords drove them out of their homeland, forcing them underground to fight with the Dwemer and go mad/blind. They were sons and daughters of Skyrim too before.
4. Another info: In one of Skyrim's ingame books (might be spoilerish), the Thalmor writes: Ulfric Stormcloak was fed with misguided info about the Empire. He's a very good asset. He will weaken both Skyrim and Cyrodill.
So who's with me?
Death to the Stormcloaks. Death to the traitor Ulfric !