[Porsche-O-Phile]

It flies "backwards" so it's in position to fire its OMS engines for descent & re-entry. The OMS engines are the two little ones on the "pods" on either side of the tail - IIRC each one produces about 6,000 pounds of thrust. These fire opposite the direction of motion to slow the orbiter enough to decay its orbital speed and cause it to begin to "fall" back to Earth. It could fly in any orientation as has been said, but I suppose having it in that position as a default is kind of a precaution in case of an RCS system failure (Reaction Control System - the "manuvering jets".