What it all boils down to, as I've said before, is that authors are artists. Nobody begins writing novels for money. When given free rein, artists burn with the need to tell a story that starts deep inside of them and eventually cries for release. As they write, they immerse themselves in the story world, recording what happens in their imaginations just as a journalist records what happens in the real world. Both journalists and artists seek to tell the truth; they just write about different kinds of truth. As a writer myself, I can tell you that this truth-telling is as real for the artist as it is for the journalist, and it cannot be stifled without causing damage to the tale. Thus, if an author "hears" a character swear like a sailor or "sees" a character act promiscuously as a natural movement in the course of the story, then that is what the author is compelled to record.
When it comes to moments in the story when the author must consciously decide how explicit to be with violent, sexual, or other controversial content, publisher limitations naturally take precedence. Outside of that, however, most authors likely follow their own instincts for what is appropriate and true. They don't want to upset parents (though I suspect a few secretly sneer, "Bring it on, Mom and Dad."). Our concerns just aren't their concerns. Their primary concern is telling the unvarnished truth of their story, however difficult or dark the material may be.
As parents, we may feel that YA authors bear a unique responsibility towards their readers that adult-market authors don't have. We feel they should join with us in helping to guard our teens' hearts and minds against content that they should not experience for at least another few years. They should help us parents protect what little innocence today's teenagers may have left (and for some teens, a lot of innocence). Perhaps this is partly why some parents feel it is right to attempt to ban and censor certain books from schools and libraries (another blog topic for another day).
We parents need to check ourselves when we place these standards on today's YA authors, though. It is hard for me to say that, because I don't like it, but it's true for a free society like ours. Even if we are morally right in our opinions of how YA authors should handle mature content, we cannot lose sight of the fact that YA authors are not gatekeepers in partnership with us.