If Fox spins the fact to sell war, why would spinning the facts to sell peace be morally wrong?

Because that way lies insanity - it also undermines any arguments against Fox's bias. In fact it becomes even more problematic; if both sides are twisting the story then that risk legitimising Fox's stance. That is dangerous.

Morally it would be... complicated but could be done in good faith. Logically it seems a poor choice.