...violates the spirit of the game to use the same armies to attack in two different directions the same turn
That's... why it's called
multi-attack...
Remember, this game is based on Risk. Multiattack is a feature made to emulate attacks in the closest way to Risk as is possible, when the turns happen at the same time instead of alternating. And if you've played Risk, you'll know that attacking different directions with the same armies from a failed attack has always been allowed.