If people drive to DTD with the intention of spending $$$ AT DTD, and they can't find a parking spot because the lot's full, and they have to leave, then anyone who parked at DTD to shuttle to a park via resort buses effed those people over, as well as the shops, restaurants & other vendors who lost money because those potential shoppers couldn't park.
I'm of the opinion DTD should charge for parking, with the more modern meters that provide a receipt you have to leave in your car that shows the expiration time, with a 6-hour maximum window, $500 fine and/or towage if you're caught with an expired receipt .
The 6-hour window is enough time for most people to shop & eat or see Cirque (and, of course, they can always return to the lot to pay for more time). But it will discourage a lot of people from parking at DTD to avoid theme park lot fees. (Free parking for resort guests, ala every other Disney lot.)
But to compensate most Disney guests, your DTD lot receipt could be used the moment it expires (with a 30 day window) as a sort of gift card in any DTD or resort shop or restaurant. This, in effect makes the parking free for just about everyone. The lone exception would be people spending their last moments at WDW in DTD before heading for home (hey no system is perfect, and since it's all just me spitballing anyway, maybe people can mail parking receipts in when they get home in exchange for Disney dollars).
Well, if parking at DTD is still in effect free, what's the difference between the current situaton and my hypothetical? That 6-hour window. It's "reasonably" easy to get back to your car to leave or pay for more parking if you're at DTD. But if you parked there for free while shuttling to a theme park, you're relying on the schedule of Disney's transportation to get you back in time. Depending on the park you visit, it might take you an hour to get to a park and an hour to get back. Suddenly your 6 hour window shrinks to 4 hours. If you intended to get to a theme park early, before it opens, your window of actual them park family fun time shrinks even more. Some people might like the pressure, during vacation, of checking their watch & sweating bullets, having to forsake long lines or long attractions, or avoiding a parade to wait for a bus to hopefully get them back to DTD in time. Some people, but probably, not many, especially if Disney develops a rep for REALLY enforcing the expiration time.
There's another perk: DTD should have security in parking lots anyway. Checking receipts & enforcing the time limits, while keeping the lots free of dealers thieves or gangs, helps cover the cost of hiring those security guards in the first place.
The only other additional factor I'd add to this is for the buses to deny non-resort guests use of buses - or rather, JUST the buses from DTD to a resort - until after all the parks open. Why else would someone go to DTD in the morning, before the theme parks open, besides a free-but-perhaps-lengthy shuttle to the parks? After all, if you have breakfast ADRs at a resort restaurant, you'd just drive to the resort, right? OR drive to the theme park du jour, bus from there to that resort, then bus back to the park, right? No need to park at DTD to go to a resort for breakfast if you're planning to go to a theme park afterwards, right? And if you REALLY WANTED to look around DTD for a while before breakfast, bus or ferry to a resort for breakfast and then go to a park, you still can do that, you're just paying for it, you've got 6 hours (minus the time you're waiting for the parks to open from the time you've arrived at the lot), and then you've got the receipt to use as cash afterwards.
That's my suggestion anyway, FWIW. I await the picking of nits with baited breath.
