I am not handicapped, nor are any of my traveling companions, but I understand why Disney loads the handicapped line onto the buses first. It's easier to load a
scooter/wheelchair onto an empty bus than to maneuver around other passengers, or have to ask other passengers to move to make space for the scooter/chair. Also there are only one or two spaces for a scooter or chair on each bus. It's not as if the bus will completely fill up from the handicapped line and no one else will be able to fit onto the bus. People using scooters or wheelchairs probably end up waiting longer in many cases because there are only one or two spaces available to them per bus. I would much rather wait an extra 15 minutes for the next bus than be handicapped and have to use a wheelchair or scooter at Disney.