The shape is designed to put even the smallest fillies and colts at ease. This invention allows earth ponies and unicorns a cheap, and safe way to experience the joys of flight, with highly skilled pegasi ready to save anyone if anything were to go wrong. This was only necessary once so far, out of the thousands of trips it has flown.
Despite what the appearance may suggest, pone-air possesses no consciousness, all movements are controlled by the pilots. There is an auto-pilot function, but that only keeps the plane on route in the air.
I dunno whose idea plane ponies were, I just wanted to draw something robotic and then I remembered I saw them on EQD at some point, so this happened. Speed and capacity is based on actual planes.