Grand Theft Auto: Chinatown Wars' 18-and-up rating isn't anything new for the series. The Japanese software ratings board CERO shows all GTA titles as Z-rated, including the two PSP entries in the series. But no other DS game has managed to secure that behind-the-counter letter rating.
Perhaps that's why Cyberfront, not Capcom, is publishing Chinatown Wars in Japan. Typically, the Japanese distribution of Grand Theft Auto games is handled by Capcom, often to success. But for Chinatown Wars, the publisher of the PC version of Grand Theft Auto IV will be taking the reins.
Maybe Capcom was turned off by the atypically moderate sales of Chinatown Wars in the face of releasing a Z-rated Nintendo DS game. We'll see if this is indicative of a trend when the PSP version of Grand Theft Auto: Chinatown Wars is released for the PSP in the country.