Many teens are afraid of what their parents will say if it is known that an abortion might be necessary, meaning that a pregnancy is in process. So making them get that consent is going to hinder many of them from really dealing with the pregnancy and is not in the best interests of any pregnant teen, but especially anyone whose pregnancy is the result of rape.
A woman who has been raped has had control of her body forcibly taken away by the perpetrator, and pregnancy only compounds the loss of control the victim has experienced at the hands of the perpetrator. For this reason, only the rape victim can decide whether an abortion is necessary in order to regain some level of control over her body. The parents were not raped, thus it is not their decision to make.