Because they are their
parents. With a minor parents should know if they're on birth control or are having sex.
I would be livid if my sister took my son (who is 7 months old so that's not going to happen

) to get BC without my knowledge. And if I did that with my niece or nephew my sis would have my head and I wouldn't blame her.
Teenagers are
not short adults. No matter how mature they may seem they lack the capacity to make those kinds of decisions.
What if I helped my niece get bc without her mother's permission and then later found out she was sleeping with a 30 year old?
While I love my family I would never undermine a parent's authority. Should my niece ask me to help her clandestinely get BC, I would suggest that she talk to her mom and I would mediate if need be (although I would prefer to wait until marriage). But I wouldn't go behind a parent's back.
Just my $.02