They're limited the amount of the item you can resell. If the package you bought came with a one time DLC code, when you go to resell the package you bought, you can no longer resell the entire package.
If there's no used games, there's less money for the consumer to buy more games with. When making a purchase, many, MANY gamers consider the resale value into the actual price of the game. If this option is not present, it will instantly raise the perceived price of the item to those people (and thus, lower the value of the product).
Further, it limits the longevity and legs of online heavy games as less and less copies are in circulation, until the game is finally discontinued and online is left to die. This lowers DLC sales and ultimately makes people less receptive to a sequel, as less people are playing the game (or have played the game) in general.