I would assume the parking issue is why the DTD buses don't go to the parks. Disney knows that short of installing meters and making security mark tires, they can't stop people from parking there for free, but they can deter a lot of it by adding the extra step. My feeling is that if anybody is willing to take the time and hassle to go to a resort first and then the parks, then more power to them. I don't think Disney is automatically entitled to your parking dollars just because you're in the parks and not staying on property.
As for the buses being only for resort guests, that's just not true, since anybody can hop a bus to MGM or AK from another park. They may say that the RESORT buses are only for resort guests, but they know that in reality, people have all sorts of reasons to go to the resorts who aren't staying there, such as meeting friends, eating dinner, just exploring the hotels, etc. Until they start requesting ID's , I have no moral hesitation to hop on that WL or AKL bus.