It seems that the way Disney does it, they want you to have an opportunity to tip twice: once when they unload your bags, and then second when they bring your bags up to your room. I stay at a very nice resort in Asheville, NC, and they unload your bags, and there is no tipping. Then, when you are checked in, the same guy brings you and your bags up to your room. I guess that would not work at Disney with the volume of guests, and the fact that many people unload their bags much, much earlier than they are going to be able to get into their rooms.
I do have a question though: When I have checked in and my room is not ready, and they tell you that you will have to wait to come back to the resort, contact bell services and then wait up in your room for bell services to deliver your luggage, I have asked, Why can't bell services just deliver my luggage to my room when the room is ready and my luggage will be waiting for ME when I get back from the parks? (This was back when we first started going and we would tip the unloading bellman 20 dollars for a couple of bags.) Well, I was told that the bellmen ARE NOT ALLOWED TO ENTER MY ROOM AND LEAVE MY LUGGAGE WITHOUT MY BEING THERE. Then, why if you are using Magical Express, are the bellmen allowed to leave your luggage without you being there???? Anyway, after we saw how it was with the 2 different bellmen, we started tipping maybe 10 dollars for the person taking our bags from the car, and then tipping 20 to the person who brings our stuff up to the room and shows us around and gets us ice.
Even though I wish it were done a different way, I was always taught that you tip anyone who handles your bags.