So, you are saying that you CAN put your bags on the bus (if you are a participating airline), you will drive the rental to the airport, and you will see your bags at home. Why would they allow bags on the bus for the return trip, without your being on the bus, when you can't do it for the arrival? This is makin me loopy!
I have no inside info on this, just applying some common sense with a touch of cynicism.
DME is a big money maker for Disney. They entice you with a free ride from airport to resort, and throw in the added convenience of the automatic luggage delivery from airport to resort. How does this help Disney? It basically strands you on Disney property. If you rented a car to get from airport to resort, you then have the easy freedom to go off-site for meals, souvenirs and other entertainment options. You've also spent money on the car rental itself, and you now have less money to spend on Disney property.
It's the same, to a lesser extent, for using a car service. You have less money to spend at WDW because you already spent some on the car service, plus you can request a grocery stop rather than paying for some groceries at your resort and/or eliminating some meals you would have purchased on Disney property.
If you take DME, you spent ZERO before getting to WDW property, so you have more to spend there. AND, you have no easy access to the outside world, meaning that every cent you spend will go into DIsney's coffers.
Disney's stated rule is that DME can't be used as a luggage-only transfer.
WHY make this rule??? Simple! If you're not on their bus between airport and resort then you must have spent money to get to your resort some other way. You have less money to spend in Disney, AND you have access to spending additional money in the outside worls rather than Disney. Since you eliminated your profitablility to them (in this regard and in this regard only), they don't want to then foot the bill for your luggage delivery. They want to tell you it's all or nothing, hoping that you'll go for "all."
Now, the way back to the airport is a different story. They don't give a hoot how you get back to the airport. You're at the end of your vacation, you've already spent every dollar you intend to spend, and you no longer represent a profit center to Disney. They'll help you with your luggage (if you're on a participating airline) for free, but if you decide NOT to take their bus that makes it cheaper for them, so they really don't care whether you take the bus or not.
I hope this perspective helps you understand why they will let you check your luggage with them on the way back to the airport no mater how you get your body back to the airport, but they tell you they won't take your luggage from airport to resort if you don't ride their bus to the resort, also.
A pretty ingenius plan, if you ask me.