The problem is that the US needs Pakistan a lot more than Pakistan needs the US at this point. Until we can reduce our troop and equipment levels in Afghanistan to the point where we can just write them off, we need those supply lines. That's a lot of what the US is paying that money for. Pakistan knows this, and sees no reason not to use that to squeeze the US every once in a while. Because the US is so invested in Afghanistan we need Pakistan's cooperation - for now - because we sure aren't going to get Iran's.