Disney discontinued the DTD/TTC & park buses to prevent people from parking at DTD and going to the parks, thus circumventing the $7.00 parking fee. ($6.00 at the time of the change) This was the most reasonable way that we could discourage "cheating" of the system. Ultimatly, there will always be some way to cheat the system...
Folks who have ultimate Park Hoppers, that include admission to a water park or DisneyQuest DO have a right to use the DTD bus, regardless of if they are staying off-site. Most often, these are guests who are using off-site shuttle buses that do not service the DTD area.
Lastly... remember... this is DISNEY. If a guest needs to get somewhere, we're going to help them. Period. That's just the Disney way.