Well, assuming your statement is true, I’d be more curious as to why other games don’t also ban this.
Because other games are dumb.
I’ve seen what synchronised multi-boxed toons can do when they all move as one, and they are the most massively insanely overpowered thing you’ll ever see in a game. They can (and do) single handledly turn the tide of battle, and they walk all over PVE content as well.
You really have to actually witness multi-boxers in action to see how devastating they can be. I’ve seen a 4 shaman multi-boxer rip through a dozen enemy players like a bowling ball through tenpins.