I've read several posts were people mention folks "abusing" the buses; however, the buses at WDW are there to help guests get around on the property. It doesn't matter where you stay, or how you got there, but if you are on Disney property you can use the buses. So, while we (and that includes me!) might not LIKE the idea that others are using the buses as an alternative just to save themselves a couple of dollars, they aren't really abusing anything. Realistically, you could say the same thing about the boats from DD to POR/POFQ/OKW and SSR. Someone could hop on a boat and take a nice leisurely ride to any of those resorts and grab a bus to one of the parks.
Folks at the AK could jump on a bus to AKL just to get a bus to MGM, etc... Someone could catch a boat from MGM to the BWV just to go to the ESPN club, then walk down to the BWV bus stop and catch a bus to SSR, then to DD to get their car!

That's a lot of time wasted on a bus though, but I guess people do it.
I couldn't find anywhere on the Disney website where there were regulations for who could and couldn't ride. Here's all I found:
Q: What is the best way to get around within the Walt Disney World® Resort?
A: There are many modes of transportation within the Walt Disney World Resort, including Bus, Boat and Monorail. A rundown of transportation within the resort:
By Bus:
Most areas of the Walt Disney World Resort are accessible by bus from other areas within the Resort, including Theme Parks*, Water Parks, the Disney Resorts and the Downtown Disney® area. Buses travel on routes throughout the Resort and do not provide direct service between all destinations.
*Magic Kingdom® Park is not accessible by bus from Disney's Contemporary Resort, Disney's Grand Floridian Resort & Spa and Disney's Polynesian Resort. These Resorts provide monorail transportation to the park.
By Boat:
The Magic Kingdom® Park is accessible by water launch from Disney's Contemporary Resort, Disney's Grand Floridian Resort & Spa, Disney's Polynesian Resort, Disney's Fort Wilderness Resort & Campground and Disney's Wilderness Lodge. The Disney ferryboats connect the Magic Kingdom Park to the Transportation and Ticket Center (adjacent to Magic Kingdom parking).
Epcot® and Disney-MGM Studios are accessible by boat from Disney's BoardWalk Inn and Villas Resort, Disney's Yacht Club and Disney's Beach Club Resorts and the Walt Disney World Swan and Walt Disney World Dolphin Hotels.
Downtown Disney® is accessible by ferryboat from Disney's Port Orleans Resort and Disney's Old Key West Resort.
By Monorail:
Disney's monorail runs between the Magic Kingdom Park, Disney's Contemporary Resort, Disney's Grand Floridian Resort & Spa, Disney's Polynesian Resort and the Transportation and Ticket Center (adjacent to the Magic Kingdom parking lot). A connecting monorail takes Guests from the Transportation and Ticket Center to Epcot.