If these 14-16 year olds get in the habit of casual sex without any other contraceptive method, then when these implants wear off, these girls may quickly become pregnant 17-19 year olds, who may then opt for abortions. They will have been trained that sex has no consequences, so they may not be prepared to expect any risks or changes to their lifestyle.
That is, they might get pregnant easily if they haven't become sterile (permanently) from catching STD's during three years of potential promiscuity.
Not real hard to picture, is it?