What? Do you really think an exercise like bench press can only be applicable to motion in the same plane it is done in? Obviously the pectoral muscles will work pushing something away from you standing up as well. In fact, the pecs have significant deltoid movement responsibility as well. Did you think the pecs only worked lying down? And pushups, which you advocate, are the same motion flipped around so how are they any more natural in your opinion? The body position in both exercises is to allow for the application of gravity to the weight being moved so as to be effective.