... when I bought my park tickets and made ADRs the Disney staff told me that if I have park tickets I am allowed to use the bus system, NOT if I had reservations at one of their hotels.
...
Maybe this is so, but it is not well publicized. There are no warning signs, etc. And so I don't think Disney cares.
As another poster pointed out, if you want to go through all the delay to avoid the parking, well I guess that's up to you.
And in the case JT hypothets, presumably the people parking at DtD are going to the parks and have tickets. So if HRT+3 is right, they're entitled.
Besides, it is hard to think of a reason you would want to use Disney Transportation if you didn't have a park ticket. Sure you can make some up, but those occasions are rare.
For example, you stop at DtD for some shopping and you want to eat lunch at AKL. Then, because it's your first day and you're not going to the parks, you hop over to MK and ride the monorails so you get the feeling you're really there. Then back to DtD and call it a day.
How often does someone do something like that?
The only valid argument is that sometimes the parking lot at DtD fills up, and so for that reason I'd say I object. But as far as I can tell, it's not against Disney's rules.