I think the Imperials should be more concerned about it than the Nords, seeing as Tiber Septim was the first Emperor of the Empire, so he should be a huge deal. Instead he's thrown under the bus, and the Nords are the only one concerned, when the only connection they have to him is race.

That's the only thing I don't understand about the whole Talos ban. The Thalmor want his worship banned because he beat the First Aldmeri Dominion when he was alive, and he's also a Man, which the Aldmeri Dominion despise, so him being accepted as a god is disgusting to them. The Nords don't like it because he's revered as a great Nord war hero. But the Imperials, the people who should be the most concerned, just shrug it off. Why?

The empire is known to look the other way in regards to Talos Worship. Tullius also hates the thalmor for banning Talos worship.

But why would they agree to it?

They felt like they had to accept the Thalmor's terms because they were exhausted. Probably wasn't the greatest idea but that was their line of thinking.

Because after a long and costly war, it probably seemed like such a small thing to let slide. Especially since that's the one thing the Thalmor wanted first and foremost.