Thinking logically and ignoring all technical aspects, US has all the reasons in the world to ensure that the weapons it supplies to Pakistan are not used against its allies or interests e.g.
1)India is a growing economic power and market for US goods
2)India is a counter-weight to growing Chinese power in the Asia
3)Israel a natural ally of US fears growing and unstable Islamic powers like Iran and Pakistan (Yes we are unstable whether you like it or not)
4)US has always maintained a transactional relationship with Pakistan instead of a strategic one
5)US has Geo-strategic interests in the region with respect to countering Chinese hegemony, Central Asian republics mineral and energy resources, and the hub of extremism in the world.
For the last 1 decade, US interests have been centered around India/China/CAR(Central Asian Republics. Israel is its long term ally. Pakistan is only in the picture due to Afghanistan. Once a 'victory' is declared US has no reason to stick with Pakistan and will abandon us just like it did in the past. US views Pakistan as an unstable and failed state.
Pakistan is nothing but a tactical short-term ally to achieve larger strategic US objectives.
Don't forget that they still talk about our nuclear weapons getting in the hands of rogue elements. How can you then expect them to hand us fighter planes which is an effective delivery platform? Thus all these factors combined ensure that US has a number of motives to ensure the weapons it provides to Pakistan are not used against its allies or interests.
Some people are arguing that since Pakistan paid for it, we get a fully functional product. Well if that was the case, defense sales would have been an integral part of a free market based economy. You would not need permissions from DoD and various other government departments to sell a couple of missiles let alone a fighter plane.
