The intent of the free dining promotion is to fill WDW Resort rooms (mostly at rack rate) during a "slower" time of the year. No reason to extend that type of promotion to DVC members using their points/membeship to stay.
Because it's a loss leader, basically a way to sell rooms and park tickets. You also can't get it with discounted rooms or other specials and you must buy park tickets for each person. Of course I wouldn't turn it down if they gave it to me but to expect it would be unreasonable.
You can purchase the dining plan when you stay on points, with no requirement to buy tickets along with it. So DVC members who want the dining plan do get about as reasonable a deal as you could expect. I don't know the price per person but I know there are lots of threads here discussing it.