The world is is a mixture of good and bad, light and dark, engrossing and banal, meaningful and pointless. The books our teens read should contain a similar mixture. Denying the dark because you find it disturbing is denying a part of the world that exists outside of our front doors. It is the parental equivalent of pulling the blankets over your head to make the scary thing go away.
Here is the hardest thing for a parent to admit (although denying it will not make it untrue) EVERY teen lies to their parents at some point. Many of them lie about really dark and horrible things. In my own case it was rape at age 14 that I lied about. I was raped by a boyfriend, the son of dear family friends, and I felt the consequences of telling were too great, so I never told my parents (or anyone for that matter until I met my future husband). At age 14, I could have really used a book that dealt honestly with the topic of date rape. I could have used the guidance and hope that such a book would have offered. My parents would not have wanted me to read such a book. They would have judged it too dark. They would have been wrong. As Sherman Alexie said, well meaning adults continually try to protect their children from the darkness that has already infected their lives.
Parents: if you REALLY want to do right by your teens, then let them choose the books they want to read and you read them too. Use the stories to start a dialogue with your kids. Discuss the conflicts raised by the stories. Examine the choices presented. LISTEN. This is how you can 'protect' your children from the big bad world - you can show them how to face it, deal with it and get over it! YA literature is one of your best resources for doing just that.