The debate of used games has gone on for some time. In fact, it's one of the most hotly contested aspects of the gaming industry today. Gamers are happy because it lets them get games cheap and readily available. A counter to this has typically been online passes. If you buy the game new, you can use the code to access online multiplayer and even sometimes free DLC.
But, this raises the question. Are single player games viable anymore?
Single player is more viable then ever now. Multiplayer based games tend to have more paid post release DLC than single player games(maps, skins, weapons etc).
I can see your point and the sad fact is they are becoming less viable. There just is no way of making sure single player games are bought new than used. I guess the best way to make money is to add dlc missions and charge extra but that screws over the people who buy new and also might not be good for the story.