the precedent is that in this case, the safety of a teenagers brain is worth more than his right to choose to protect it, since by definition he is a minor who is incapable of making the smart decision to begin with.
1. By this logic, a teenager is unable to make any decision regarding his health, safety or well-being. So they shouldn't be allowed to drive at all. They also shouldn't be allowed to eat unacompanied by an adult since they aren't capable of making healthy decisions. Etc.
2. The argument implies that there is a correct
decision so any adult who opts out is clearly incapable of making the smart decision to begin with. Is not the safety of an adult's brain worth more than his right to make poor decisions? Therefore, adults should also be subject to this law which will force them to make the correct decision!