In my opinion, much is depends upon temprament PAF's pilots as well as entire contingent to share their expertise in Red Flag under the cloud of conistent missile attacks by US drones on Pakistan's northen borders and subsequent perishing of innocent Pakitani civilans in this attacks. Because if I put myself in place of PAF's pilots, then I had chosen to boycott such exercise offer to protest cosistent US drone attacks.
Two different things here. Why should the PAF boycott such exercises when all other government to government contacts are intact and the Pakistani forces are taking on some of the same people that the US strikes are launched against?
While there are issues with innocents dying in such strikes, its for the GoP to tackle and not the PAF to protest. They have to carry on with their professional training and exercises as business as usual.