Dalaran is neutral city,Kirin Tor proclaimed themselves neutral in the conflict between the Horde and the Alliance
Lorewise originally Dalaran belonged to "Alliance of Lordaeron" but most of those people are Forsaken now.
(Gameplaywise pre WOTLK it was just a giant purple dome. You could never get into it and you couldn't see inside it.)
In theory Jaina ruled Dalaran during MOP but for real she is not in charge.
And Sunreavers represent only part of Horde and not whole Horde.
Khadgar led an expedition to the alternate Draenor, establishing his mage tower in Zangarra. Jaina ordered Khadgar to not allow the Horde to step foot on Kirin Tor grounds. Despite this warning, Khadgar worked with both the Alliance and the Horde to accomplish his goals on Draenor.
Another factor to consider is that many Forsaken and blood elves were formerly citizens and even rulers of Dalaran.