User blog comment:Sgt. S.S./Why I support the Empire/@comment-8911126-20120329190211/@comment-1470368-20120330214358

Ulfric may be a Thalmor puppet, but...as it is also stated in the book, if the Stormcloaks do win the Civil War, then Ulfric would target the Aldmeri Dominion as well.

The Thalmor were depending on the Civil War to continue to the point that neither side would emerge victorious...So, the Dragonborn joining either side would pretty much thwart their plans.

If they ever make an Elder Scrolls VI, I wonder which side that Bethesda Studios will decide to ultimately emerge victorious...and if it'll be considered "canon" whether the Dragonborn actually participates in the Civil War.