ESRB (games rating system) is the same way. Almost no physical retailers will carry NR (Not Rated) or AO (Adults Only) rated games as a matter of corporate policy. ESRB DOES clearly list whatever content caused the game to receive it's rating though. If you're a dev who wants to make a game and distribute physical copies, you can still do so online through Amazon or some other distributor, and Steam hosts digital distributions without ESRB ratings of any kind simply because there are too many indie games for ESRB to rate (if they all applied for ratings that is).