I don't have a problem with parents choosing to let their children read books with such language... but I think Harry is right that certain expectations have been established by publishers about what will and will not be in books marketed to children. If the language described is necessary for the character, perhaps the book might be marketed towards teens instead, or perhaps it might have a "language" warning on the back, as suggested.
I've routinely let my kids read books that were considered "too old" for them, again encouraging them to talk with me if they have questions or if anything makes them feel uncomfortable. But I appreciate the heads-up that the categories give me, so I can keep an eye on what they're reading, generally (without having to wade through angsty teen romance vampire stories myself
).