Putting some thought into it, I can't imagine that the surf coaster's seating position would be prone position. With SW already having a flying coaster, that seems a bit redundant. If I was a betting man, I'd say they probably figured out a more comfortable restraint for standups and adapted it to the wing coaster.
Standups are cool and all, but I have serious doubts about my reproductive health after riding Vortex at Carowinds one too many times. :look: