DL can check with the ticket-issuing and airline-operating carriers (if they really wanted to) and find out if the appropriate ticket coupon was used/submitted or if mileage credit was given already. Obviously, it may not always be worth their while (or the other airline's time) to do that, particularly given the labor cost involved; however, to limit their being exposed to financial risks of issuing miles in such circumstances, the airlines outsource the work onto the customers by requiring the customers spoon-feed them the information.