In that scenario renting points prevents them from loosing money outside of the resort.But they're getting that anyway. Whether I decided to rent points from an owner or not, they've already paid (and are continuing to pay) Disney. Me renting points doesn't give Disney extra money.
The DVC member renting the points can obviously not go to WDW for whatever reason. If they loose the points Disney still makes money from DVC but looses money on tickets, merchandise, food, etc if they can't sell the room for cash. As you might have guessed selling a 1BR DVC room or something similar is much harder to do then standard room at the Poly or anywhere else. By letting the DVC member rent the points they have the room filled with a guest that is statistically more likely to spend more money because of the great room deal they got. To add a little icing onto that cake, the person renting might like the DVC accommodations so much more than a standard room that they buy into the DVC.
Like in Vegas, the house, or to be more correct, the Mouse, always wins.