The Empire didn't exactly surrender, they defeated the Thalmor Army in Cyrodill, completely destroyed it and hung their General off the big tower. The Empire formed a peace treaty to end bloodshed for the time being, they did not simply roll over and surrender, they lost more than half their Army and they get crap from Stormcloaks because of what? They didn't march to Summerset Isles? They'd have to retake every province before they could even attempt to go there, if they didn't then they would need to leave soldiers behind to guard the cities and borders and well gee it is sure smart to invade with a splinter force that is worn out and cut off from reinforcements by an ocean to fight against an Army in their own land? which know the area and have all the resources they need.
You get the "Well hammerfell kept fighting" Hammerfell defeated Thalmor in Hammerfell and formed a treaty with the Thalmor, the Empire did the exact same, should we hate the Redguards because THEY didn't march on to Summerset Isles?
As for Ulfric Stormcloak, he does not care for Skyrim or the people in it, only himself. You see this many times throughout the story. Peace talks with Him and General Tullius. Tullius will agree to a truce while the dragon threat remains, Ulfric the "True" High King, only will agree if the Empire gives him some land to help him in his war effort, Tullius goes alright but we get one aswell then, Ulfric complains at this the talks continue then when you think we're getting somewhere what a surprised Ulfric wants another Hold, for a man who claims he fights for his people he doesn't seem to exactly care about Dragons killing his countrymen, unless he can use it to his advantage to become High King.
The High King respected Ulfric greatly and would of went for independence if Ulfirc simply asked him. But no Ulfric killed him because he wanted the Throne. The main reason the High King didn't go for independence when Ulfric was merely hinting at it during the Moot for Torygg, not asking for it, just hinting shy of treason. Is because Skyrim depends on the Empire for trade with the food the Empire brings and other stuff with the East Empire Company to Skyrim.
Back to the Empire though, there is a big difference to simply rolling over to the Thalmor and fighting them to a stand still. The Thalmor are more talk than bite, they fear the Empire and they have good reason to fear it, The Empire has been rebuilding for thirty years, they haven't been sitting around doing nothing, their Legions are going up to full strength, there is only one Legion in Skyrim the Fourth Legion and the Empire has several Legions. So if you were fearing the Empire you'd try to divide them, limit their advance and buy time. The Stormcloak uprising was started by the Thalmor with Ulfric doing exactly what the Thalmor want, they even give 'indirect' aid to the Stormcloaks. That is the only reason the Stormcloaks are holding against Tullius.
Ulfric isn't all that great, he was captured by Tullius (Well he surrendered to Tullius without a fight. Hadvar and Ralof are at least in agreement with that.)
Be sad for the Stormcloaks if the Emperor is killed by the Dark Brotherhood as he is the only reason The Empire hasn't marched into Skyrim hitting it like a hammer to an anvil. I suppose the Emperor has his reasons probably wants to win the Nords back with a mainly Nord Legion. It is the reason the Fourth Legion is recruiting locally because Mede won't commit more forces, if he was to die in Skyrim, the Council could blame the Stormcloaks for the assassination with hiring the Dark Brotherhood, then a great purge would happen as several fresh Legions stormed into Skyrim like a flood. Ulfric may have stood against one Legion with indirect Thalmor aid but he will have a tough time against more, considering the Empire has 18 Legions all up.
High treason is high treason. The Empire turn their backs on their own people by agreeing to the treaty. It's simple as that.
