Well, they only revert the Horde to "evil" in order to immediately iniatiate their redemption arc and at the end of the day when they have cleansed themselves from the evil elements (Mannoroth's blood, Garrosh, AU Gul'dan, Sylvanas) the Horde is good and innocent again, Draenor is free and the only Alliance actors (Daelin, Jaina, Yrel, Genn, Tyrande) who disagree with this are portrayed as crazed/vengeful in order to create some semblance of "both sides are bad" (even though the plot of the game unintentionally keeps justifying their beliefs).
It's just that with every retelling of this story (I think Danuser even outright said that BfA was a retreading of MoP's story) it becomes more muddied and nonsensical. That's how you end up with character arcs like Jaina's who comes to realize that her father was wrong by seeing the Horde do everything Daelin had warned her about.