Some 17-year-olds might be mature enough to handle certain situations, and some might not be.
Like I said, I can see the good in allowing a 17-year-old to pick up Plan B if she needs it, but I also think that before a child turns 18, it's the responsibility of the parent to communicate and help the child in any way possible.
I don't think it should be that hard (but for some, unfortunately, it is) for a girl to go to her mom and say something like, "Mom, I had sex last night and the condom broke..." -- chances are, a parent doesn't want her/his child to be a mother at 17, and chances are, s/he's going to help somehow... it could be getting Plan B, or bringing to her a doctor, or talking to her about using a regular form of birth control.
Plan B is for emergency use. The majority of young girls are so afraid of their parents finding out that they're having sex, that they aren't going to think about that. The majority of them aren't going to stop and think that just because Plan B is available that they should still use an alternative method of birth control (because really, why would a girl talk to her parents about getting on birth control if she knew she could just walk to a drugstore and pick some up without anyone knowing?). A good portion of the girls who could have access to Plan B will think that they won't get pregnant because they took the pills, which isn't the case. Plan B isn't as effective as regular birth control, but that doesn't mean children aren't going to use it like it is.
I'm sure a lot of adults (18+) don't think about things like that, either. But at the same time, adults are responsible for themselves. Parents are responsible for their children. Accidents can happen whether Plan B is taken or not, and I don't see why parents shouldn't be involved in making decisions like that with their children, that's all.