Actually, for bags to "magically appear" in the guests' resort rooms, it was absolutely essential for Disney to eliminate the need for guests to tip for inbound bell services. (Otherwise, guests would have to remain in their rooms waiting for their luggage, to tip bell services, who would not deliver to an empty room.) It was necessary for Disney to absorb the cost of inbound bell services gratruities to make the inbound service work. And that's exactly what Disney did. I give Disney high marks for this.
Guests can check in and then head out to have fun (and spend money), without having to wait some number of minutes (or hours) until their luggage is delivered.
Disney had to communicate that the bell services gratuity is included for inbound luggage delivery, which is one reason it's explained in the video on the ME motor coaches. (Is this part of the video the reason that there are so many reports of ME drivers not running the video?)
Disney could and should provide tipping guidelines for other parts of the service, including the ME drivers, outbound bell services, and Resort Airline Check-in (do we tip them the way we tip skycaps?).
I disagreee with the idea that Disney should say, "Bell services, at your discretion may be tipped directly, or added to your room charges." That's incorrect. The cost of luggage delivery to the room is absorbed by Disney, as described by Tyler.
Frankly, I think a bigger reason that ME motor coach drivers tend not be tipped is that the transportation service is free. Let's see... 20% of zero is... let me calculate that out... oh, yeah... it's zero!
Also, fewer and fewer people will have luggage on the ME motor coaches as the packets including tags are mailed properly, and especially if more airlines participate in Resort Airline check-in. So, more and more, the ME drivers will be seen as untipped transit drivers, not as tipped service employees who interact with guests to provide luggage handling services.
For someone who is looking for a steady stream of tips, being an ME driver is probably not a good job. It could get better if Disney provided better guidelines, but, even then, it's going to seem like an untipped position to the majority of ME passnegers.