It is accepted fact because the lore supports it. The existence of the scarlet crusade is a perfect example. The members came from the same population pool as the humans of Stormwind.
The reason the Scarlet Crusade has a presence in the plaguelands at all is because of human hatred of undead. The undead starting area pre cataclysm is about fighting off the scourge and the humans who refuse to acknowledge a difference between Forsaken and the scourge. Alliance player raids into the plaguelands to kill Nathanos were adopted into the lore as cannon. In spite of Before the Storm and its wild departures from established lore, even BfA manages to remind us, through Lillian Voss and Thomas Zelling, that the default experience of Forsaken is cruel rejection by their living loved ones. This isnt to say I blame the living for rejecting the repulsive animated corpse of a once-living relative, with scraps of human viscera dangling from a jaw that seems to be from another corpse entirely. Thats natural. Thats what the story should be. Pretending thats not the norm is willfully ignorant.
Alliance presence in Arathi, the battle over the contested territory of Arathi Basin is further evidence of Alliance agression. If the Alliance was willing to recognize the Forsaken as living things and not a zombie horde then Arathi Highlands is the territory of Lordearon and their birthright. Hillsbrad also. Alliance internment camps in Lordearon should have been abandoned after the Third war. This isnt to say that this would have been good strategy, sound leadership nor very much fun, but the Alliance was never written to be innocent victims. Both the Horde and Alliance were written to be rival nations at war.
This is straight out of the pages of Before the Storm. In a more subtle narrative its even partially true that Sylvanas was telling them this and for ages actually believed it because that was the forsaken experience.
In a world where better writers were at the helm of BfA, a positive development of the Alliance to one of accepting the undead could have been given to us, and Sylvanas could have refused to accept it, leaving the Forsaken in a huff, but the way that was attempted was horrible, hamfisted and forcefed. Instead of acknowledging the Alliance rejection of the undead people of Lordearon, setting into motion the conflict in the Eastern Kingdoms and the acceptance of the Sindorei into the Horde, healing old wounds and moving forward in peace under the leadership of Anduin, writers like Golden whitewashed the entire history of the Forsaken, painting them as the unwitting pawns of Sylvanas (again partially true) fighting an alliance bogeyman that never really existed (conpletely false).
All the flavor text of the lore aside, Sylvanas rightly predicted that without Darnassus, the Alliance would move to take the Undercity. True, it was Sylvanas order to burn Teldrassil that created an excuse for the Alliance to move against the Undercity, but the Alliance wasn’t there to kill Sylvanas. They were there to move against the Horde. The burning of Teldrassil was only the most recent reason for the Alliance to hate the Horde, and there was no reason to think that any other reason humans, dwarves and nightelves, Gilneans and Dreanei had to exterminate the Horde as individual NPCs or even PCs would not be enough motivation for the Alliance to move in the future under less pacifist leadership. Varian taught Tyrande patience in Pandaria, only so she wouldn’t be led into a trap by Garrosh not because he wanted to save horde lives.