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Free dining" is a promotion used to sell rack rate room/tickets packages at the historically slowest time of the year for WDW and it has been a smashing success.
DVC doesn't need to offer its members free dining. If you're booking on your own points, you don't need to pay extra for the room. If you're renting points from a member, you're likely getting a substantial discount for the room. So there isn't any need for WDW to throw in the dining plan as an incentive to get you to book.
As posted, you can still purchase the dining plan for a DVC reservation. It's already a great deal because of the price of the room if you're renting, or the fact that you're using your own points for the room and have no extra cash outlay to make. Not only that, but there is no requirement that you buy park tickets.
If you book a room in a DVC resort with cash through CRO, you can possibly book a package with free dining, if it is available at that resort for the days you book. But you will be paying rack rate for the room.