This problem is very minimal compared to the problem with those from parks riding SSR busses to get to DTD.
I don't know if the problem is comparably minimal. Quite frankly, I'm not sure how prevalent the SSR problem is. I've stayed there twice and while I only used the buses infrequently, I never heard anyone specifically say they were cheating the system by walking there from DtD. Conversely, I've never heard anyone make similar claims in any other resort where I stayed.
Then again who knows... maybe to get to the parks they do what you are saying but to get back they ride the SSR bus?
If I were the freeloading kind, and were in the parks at the end of the day, I'd a: walk from MK to the Contemporary or monorail to Poly or GF before taking a SSR bus to DtD. Those buses would be packed with SSR guests (and dumber freeloaders), whereas there would be comparatively fewer people at a monorail resort looking to go shopping at DtD at that time of night. Things were probably different back when there was Pleasure Island to visit late night, but now, I can't imagine a whole lot of people, staying at monorail resorts, who'd want to see a movie or dine at DtD after closing the MK (when most DtD will be closed or closing), meaning only the "smarter" freeloaders are probably on the buses with me.
2: Take the bus or boat launch from Epcot or the Studios to an Epcot resort, then bus to DtD from there. Same reasons as above. By taking the walk or launch firs thing, I'm avoiding the crush of people looking to take the bus back to SSR, I'm not worried about having to stand - after being wiped from my day in the parks, only to then have to walk from SSR to DtD after that (or wait for a boat).
Which again leaves Animal Kingdom & the water parks, all of which tend to close earlier than the other 3 parks. And because they close earlier, it's just as likely anyone going from those parks to SSR is also staying in a different onsite resort, and willing to deal with the crowds of SSR guests and the freeloaders to get to DTD in one shot (though I'd probably "beat the system" by going to Port Orleans FQ or Old Key West and boating from there. After all, there's still probably plenty of time to visit DtD that time of night).
Either way, there are WAY too many non-SSR guests/owners filling up space on the limited SSR busses on the way back from the parks in my opinion.
Maybe, but your evidence is still anecdotal, and you have no evidence - anecdotal or otherwise - that it's not a problem anywhere else. The freeloaders bother me too, in as much that they seem to be getting something for free. Granted, a ticket to a Disney park grants you unlimited use of Disney transportation, but they seem to be infringing on the transportation you seem to be paying extra for by virtue of staying at a Disney resort at their higher rates. I feel safe in writing that, for all the questionable decisions made by parks management, they know that people are parking at DtD and using alternate means of transportation to get to the parks, just as they know people take old mugs with them for refillin', and park at the Contemporary and walk over to the MK, and pool hop if the lifeguards don't check for resort IDs, etc. etc. And if they're not taking the steps to correct this outrageous abuse of their amenities, they've decided it's the cost of doing business, a cost worth eating, rather than come up with a solution that might cost them money elsewhere in the long run (for example, charging for parking which would cut down on those freeloaders, but also infringe on bona fide shoppers who might take offense at having to pay as much for parking at a glorified strip mall than they would at a theme park, unless they validate).
So for now, the best thing for you to do is what other have suggested, write your Strongly Worded Letter to Disney. Let them know how often you experienced non SSR guests, people who weren't even staying in a Disney resort, using the SSR bus line. Let them start to think about how much they're losing in parking lot fees AND a loss of potential repeat guests (if you're that upset about it, would you go back?). If they get enough complaints, they'll start to think that it really is starting to eat in to their bottom line, and do something about it.