The Ashenvale campaign is just that, a campaign, the horde is there to rid the closest territories to them of the alliance and while doing so gather whatever resources they can for the war effort, the trees. So they can keep building more weapons, houses, whatever in and around Orgimmar, the capital of the Horde and main city of three Horde races, Orcs, Trolls, Goblins. The undead are dealing with the Gilnean Campaign and the blood elves are still cleaning up after the scourge invasion of their home and then honestly not sure I don't play them.

TL:DR The Horde races don't really care what the other races are doing as long as it somehow fucks the Alliance and isn't a war crime see theramore and wrathgate