What surprises me is that no one here has mentioned how GameStop intentionally makes used games such a huge issue for developers. First of all, their pre-order system is designed so that they know ahead of time exactly how many copies they need to buy from developers. Has anyone ever walked into a GameStop to buy a new game and been told "Sorry, we only have enough for pre-order customers?" This is not an uncommon phrase in that store.
The reason GameStop manages their store like this is it means they buy exactly as many copies as they need for their pre-order customers. No more. In any other business, you buy from the creators of a product as many as you think you need, then restock as time goes on. You do not tally how many people need toasters and buy exactly that many. That would be outrageous, as people do not generally know ahead of time when they will need toasters. GameStop buys exactly as many as they need, once, and then never have to restock. They simply buy back from the customers and resell to other customers. From this, they can minimize buying from the developers, and send as few games into circulation as possible. GameStop, unfortunately, gets away with this by being in essence, a monopoly. There is no other big company that specializes in selling games.
In a nutshell, the more games there are in circulation, the more money the developers make. For GameStop, the FEWER games there are in circulation, the more money THEY make, because they're just reselling the exact same copy over and over and over again. So we have conflict between the creators of our games and the salespeople who sell them to us. GameStop is working very hard to sell as many used games as possible and as few new games as possible. And they're doing a very good job. That's scary for developers, because they see little they can do about it. So they resort to finding ways to punish players for buying games used. Well, customers don't really like being punished for buying a product in a way a faceless corporation doesn't like for them to, so that sure as hell isn't making their own customers happy.
At this point I'm getting into topics that are discussed in a much more thorough, organized and thoughtful way in that episode of Extra Credits other people here have mentioned. I'll just post a link so you can have it right away.
http://www.escapistmagazine.com/videos/view/extra-credits/2068-Project-Ten-Dollar