Why would Microsoft really care about used games on their own? Most of the software/games they make are for PC which have their own types of protection for their copyright. They make a limited number of games for consoles and the games they do make, specifically the Halo franchise, are more or less just designed to sell more XBoxes. Their business in the console arena is in console and accessories, not in the games themselves. The push for no used games is likely coming from Activision, EA, Ubisoft, etc. who are having their profits cut into by used games.
What I don't get is why the software people don't just sell limited licenses with codes and online verification the way they do with PC software. If they did that, it would become illegal to sell "used" games since only one license was sold in the first place so places like Gamestop would have to stop selling them or face criminal and civil penalties.