They started doing this quite a while ago; while it generally isn't the best value for obtaining miles, it's in some cases cheaper than using points.com to buy the miles outright, because you do not pay a transaction fee.
If you have more than one person in your PNR and each person has a different status level, you'll see different prices for each person; the CP pays more, the general member pays less, because there are different levels of elite bonus to multiply and the more miles you end up with, the more you pay.
Personally I don't take advantage of these offers; I just don't see the value. But if I were a few thousand miles short of an award, I might reconsider.
I did a few tests last year and found that any Mastercard, DPQM or other promo's do NOT stack with the DM Multiplier (which is a bummer, because if they did stack, and DMM was multiplying what MC gave you, then it might be a great value), but they DO stay in your account if your itinerary is cancelled. It's handled as a separate transaction and usually hits the account fairly quickly so if you need miles in a super hurry, booking a ticket somewhere that gets you 1/2- 1/3 of the required miles, then canceling the ticket within the 24-hour free cancellation window, might be a fast and cheaper solution (although still not really cheap, as the per-mile cost is between 2 and 3 cents in the routes I checked).