The most plausible theory I've heard relates to a lawsuit a while back trying to force to use of segways in the parks as a form of assisted transport like wheelchairs and scooters.
My guess is that Disney doesn't want to look bad from a legal standpoint of banning segways, yet using them for in-park tours. I've got a feeling outisde park tours will continue, but we won't see any segway use inside any of the parks.
Are security and cast members still using segways inside the parks?