It's so more graphics can be seen on the seatposts - you know, the little height notch markers.

But more seriously, do you think that a smaller frame could aid in more maneuverability? Perhaps the riders on the smaller frames feel they can zip around on it better in tighter corners and the like - I don't know. I know in mtb'ing and sloping top tube frames in general, the longer seatpost extension does look cool - but again this just comes down to looks and not function.