Plan B is safe if taken as directed -- as an emergency contraception on an occasional basis. Since birth control pills are cheap and available for women over 18, despite needing a prescription, we have not worried that women will substitute Plan B for birth control pills because there is no need to do so and because we anticipate they will understand the warning label wrt potential consequences if they use Plan B as a substitute for regular birth control pills.
This is demonstrably not true for pre-teens and young teens. The entire point of waiving the prescription requirement is that the need to get a prescription is a barrier to getting the medication. Since birth control pills require a prescription, we can assume that there are young women who would use regular birth control pills, but do not because of the prescription requirement. Why do we assume that they will not therefore use OTC Plan B as a birth control pill?
The rational thing is therefore to make regular birth control pills equally accessible. This will prevent what could otherwise be a dangerous substitution.