Language in YA

I recently took a chance on a book with a 14 yo protagonist. I don't swear or appreciate swearing in my entertainment, but I rationalized his swearing for the first couple of chapters because the boy had just lost his mother, been dumped into a bad foster home, etc.
But I eventually decided to stop reading it.
I don't understand why swearing and/or promiscuity (this book was thankfully free of that) are considered "adult" behaviors and forced on child/teen protagonists.