It's legal, so there's no problem there.
If we look at it from developers perspective each used copy sold raises the price of used copies which means they can keep the price of new copies high and continue to ship new copies as well at fulll price. Not good still but it's not as bad as piracy.
Not only that but if people knew from the start they couldn't sell used games they may not have bought it in the first place causing lower sales. In fact in Japan the market might collapse if they couldn't sell their games. Rentals serve a similar purpose, if rentals and used game sales were both gone sales would go down significantly especially for new IPs and single player games.
currently playing: Skyward Sword, Mario Sunshine, Xenoblade Chronicles X
