Except that the Thalmor and the Empire aren't one entity. The Empire doesn't want to ban Talos, many in the Empire likely Worship him as well.
However the Thalmor are very powerful and if humanity doesn't surrender peacefully then they'll come in force and silence them forever. The Empire is being smart, they're playing the waiting game. Lay low and recover to fight another day, but that will only works if they keep the numbers on their side.If Ulfric wins, then what? The Thalmor will decide that they need to finish them off personally. The Nords will be destroyed and the Empire can only watch. Of course they could come to their aid but with their current losses that would be foolish and they both end up being destroyed. Ulfric is a fool and his situation is totally different from that of George Washington, for Ulfrid victory also means destruction but he is too stupid to realize this. Which is why I love joining his cause with my Orc

I think Bethesda has created a good scenario in which there is no clear good or bad, even if you look at it from one perspective (like you did). This means that if you roleplay you can find a good reason to join either side as well as find a reason to hate the other with a passion.
Sounds like something Chamberlain would have said in the '30's. The flaw in this kind of expediancy is that the Thalmor are not going to sit on the sidelines twiddling their thumbs.
They're getting stronger too. More entrenched. Unless you beleive that the Thalmor are so stupid they don't know that it's an Imperial ploy to buy time.
And the people of Skyrim will get used to enslavement, their beliefs being outlawed, and...what's next? Maybe Nord costumes or other traditions being banned--the English did that to the Scots after Culloden.
Ulfric realizes that if humanity doesn't come together to fight the Thalmor...now...it never will and the dire predictions of calamity will come true just as surely, only more slowly, more insidiously.
Placating implacable enemies never works.
Ulfric represents a tentative and fragile impulse to unite all Skyrim against the Thalmor.
The Empire represents a well entrenched impulse to sit back and enjoy your temporal comforts while passing ever-worsening problems onto your children and grandchildren. Stasis.