Frankly, I don't know. This is exactly what I am trying to discover at the moment why in such positions uke has an inherent weakness. The palm up example has many applications seen in many techniques. The thing is it is not even technique but alignment/position of the body. If the body has inherent weakness by such positions then I would believe the opposite holds true that there are positions that the body would hold inherent strength. Sorry but I don't know what goes on in the body why it has such an inherent weakness.
I don't know about the specific example you gave, but coming from a Tohei lineage a lot of things happen "palm up" and IMO the reason for this is that naturally turning your hand palm up, adds a feeling of structure/connection to the arm that isn't there palm down in the average person and that added structure makes doing some things in aikido feel easier/better. Now imagine you get that feeling through your whole body and can feel that way without having to "palm up".