Financially speaking, there's absolutely no reason that Disney would want to do this.
When you get the dining plan, you're getting a significant discount on food. Disney recoups the cost of that discount by charging you the highest possible rate for your room. That's why you have to get a package in order to get the dining plan, and why they don't allow any room discounts in that case.
So, Disney's not really giving anything away - they give you a discount on the food, but get at least some of that money right back from you when you pay for the room.
If they give the dining plan to people who aren't staying at Disney resorts.....there's no way for them to make back the cost of the plan.