You say the Nightborne "betrayed their Alliance helper" as if that's canon.
Individual players aren't the canon hero, they're part of a quantum-shifting Protagonist cut out that performs actions Blizzard can attribute to any NPC they want to. Chances are pretty high the canonical hero who helped the Nightborne will turn out to be a Horde, and the Lightforged will have been helped by an Alliance to balance it out.