I understand how the buses work but I also would find it strange that none of them would be going back to any resorts in the morning.
There isn't a huge need. Here's a common enough scenario. A guest is staying at the Doubletree, in DTD. They don't want to use the shuttle service provided by the hotels...tales way too long, but don't want to drive and pay to park at a park. So, they hoof it over to DTD Marketplace. They want to get to MK, so head for the CR bus stop. It's now about 8:00. A bus from CR 'may' come along within 15 mins or so. When the family walks up to board the bus, the driver tells him he isn't going back to CR. He may tell the family that he will let dispatch know a bus is needed. Or, there could be transportation CMs there, working, that will call for a bus. But......that bus may very well take another 20 mins or so. DTD isn't a hugely popular place early in the day, is it doesn't make much sense to have buses ru Ning to, and from, all the resorts, every 15 mins. Even at the best of times, bus service to and from DTD is very time consuming.
Disney doesn't much care about making it convenient for those staying off site to get to the parks...using DTD as a jumping off spot. That's just not how the system is designed to work. Buses are dispatched where they are needed the most.
When a choice to stay off site is made, it should be understood that some things just aren't going to be as easy as when staying on site. You may pay less, but get a nicer/bigger room/suite in non-Disney resorts. But you trade off ease and convenience.
As far as who is allowed to use Disney transportation? Any guest holding a park pass of any kind. Yes, if you look at the resort pages, the info states resort guests can take transportation to the parks....but nowhere does it say that 'only' those staying at WDW resorts are 'officially' allowed to use transportation.