How would you get it in gear if the input shaft was turning? Think about it, with a clutch assembly, the only thing that grabs the input shaft is the clutch. It is the only component that has splines. The tip of the input shaft sticks into the back of the crank, but there is a bearing there (be it brass or roller, either way, it doesn't grab the input shaft). When the clutch is disengaged, it is not spining, therefore neither is the input shaft.