I think where a lot of the issue comes in, is that back 30 some odd years ago, when busses first started, a guest staying on property DID have to show a resort ID to board the busses.It was a perk for staying on property, as mentioned before,like the DME busses are today. I think, though I cant swear to it, that the monorail at the time unless you were staying at either the Poly or Contemp,(Grand Flo wasnt built yet) was ticket ride. IE you had to have a ticket book with the A,B,C rides etc.Poly and contemp guests got the monorail service as a perk.
As time went on and things evolved, the purpose of the transportation on property changed, the signage hasnt.
At one time, when I was in bus ops,and the parking lot of the MK there was different levels of park closings, level 1-5.(I think that was the levels)It generally applied to the MK as it was and is the most frequently closed large park due to attendance.
A level 1 if I remeber correctly was that the parked closed to day guests.Anyone already in or had a hand stamp, plus the passholders and resort guests were the only guests allowed in.
A level 2 was that only resort and pass holders were allowed in. Day guests were sent to Epcot
A level 3 was resort guests only
A level 4 was the park was closed to everyone.
Dont remeber what the 5 was or even if there was a 5.
Under this system, when the MK was at a level 3, a day guest at Epcot, could hop the monorail, and get to the gates of the MK. To help with this, it was required of transportation cast members to ID everyone getting on the sytem to avoid a day guest gettting turned away at the gates and causing unwanted issues.
Keep in mind that this was 15 years ago when I first worked at the MK parking area, so I may have the levels off or even backwards.
I think that they have pretty much done away with that system. Now if I am not mistaken, when a park closes for capacity, it closes to every one for several hours.So, no more checking IDs. Even though some of the older bus signage may say "Must present a valid resort ID" Those are being changed as bus stops undergo renovations.
Truth be told, the transportation at Disney is "Free" but not "Free"
It is free in the sense that you dont have to pay up front as you would in a normal transportation sytem in any city in America.
Its not free in the respect that transportation gets a certain amount of money per ticket, pass, and hotel served.Last I heard, each resort on the resort line paid something to the order of $650 per hour per monorail that serves that resort. (that was 10 years ago) Busses and watercraft and now the trams that serve the parking lot all get monies from the resorts and general ticket funds to pay for the service.Trams/parking money comes from those guests that drive a car,and pay at the toll plaza and also from the general ticket monies.
In the end, you are paying for the service, you just dont know it.Like mentioned before, Disney doesnt care how you get from point a to point b, as long as youre spending money along the way.