Here is what it says in the current edition of the Downtown Disney Guidemap:
Leaving the Downtown Disney Area?
* To get to Magic Kingdom Park, Epcot, Disney's Hollywood Studios or Disney's Animal Kingdom Theme Park, take a Bus to any Walt Disney World Resort Hotel, and then a Bus to the appropriate Park.
* To get to Disney's Typhoon Lagoon Water Park, take a Bus to any Walt Disney World Resort Hotel.
* To get to Disney's Blizzard Beach Water Park, take a Bus to any Walt Disney World Resort Hotel and then another Bus to the Park.
As you can see, Disney is telling people to use Resort buses to get to the theme parks. And while not mentioned, it's obvious that you can get back to DTD using the same Resort buses in reverse. Everyone seems to assume that Disney frowns on this practice. If that were the case, they wouldn't print the instructions on how to do it!