Ashenvale was described in the book as the most populated area of the night elves, maybe it is simply underrepresented in the game. Also it acts as a defensive buffer for Darkshore and Hyjal. Allowing Horde to be there is like purposefully exposing yourself to the attack.
And still i see the problem here - night elves already made peace with the Horde before, after Pandaria. And they had all the agreements and such. Which all went to the shitter anyway when they Horde decided to attack. They even had status of Ashenvale settled back then, with Horde withdrawing all claims on it.
See how it ended?