I've said this before - the Afghan's can't afford to play this game.
Pakistan, with many magnitudes more resources, a much, much stronger, disciplined and better funded military and intelligence, with decades of institutional memory and experience from working with the CIA against the Soviets in Afghanistan, still struggled to contain the domestic fallout (terrorism and terrorist attacks) until the decision under Sharif & Sharif to clean out all of FATA.
The Afghan's can't even control large swathes of their own country so their support for terrorist groups like the BLA, TTP etc is going to result in far more carnage being wreaked in Afghanistan than any damage they could inflict on Pakistan.