But.....that transportation isn't 'free' to the S/D..they pay for it and they pay quite a large amount for it. I'm sure that Disney would charge them an arm and a leg to add DME to their services. DME is one of the 'selling points' for staying at a WDW owned resort. SoG doesn't offer it either..and that's for our armed forces folks..I would think it would be offered to them.
Perhaps, at some point, DME could be added but at a price.
Don't forget that the S/D both charge for parking...about $12 a day, could be more. That adds up in addition to the cost of the rental car.