Think of them as a movie company that does R-rated movies or a television writer like Aaron Sorkin, who develops adult tv shows (SportsNight, West Wing, etc).
While some devs do "OMG M4TUR3" for the sake of it (hello BMX XXX), Rockstar targets the rapidly growing adult gamer market with adult-themed games.
See the difference between shows like Drawn Together and NYPD Blue.