Thats common enough that there's even a legal definition for it. Estoppel. If the people charged with deciding whether or not it's legal say it is legal, then it *is* legal, even if someone changes their mind later (from that point on you can be charged, but not while holding the estoppel "get out of jail free" card).
... you're wrong. Estoppel stops people from changing facts... it cannot be used as a get-out-of-jail-free card for preventing prosecution.
Prosecutorial discretion is not governed by estoppel, because (1) the decisions are made upon each individual case, so just because a prosecutor declines to prosecute in one case, does not mean that he must decline to prosecute a different case with similar facts. (2) just because a prosecutor declines to prosecute, he does not make any statements of fact as to if something is legal or not. In fact, a prosecutor can make every assertion that something is illegal, but he is simply unable to prosecute due to for instance, lack of evidence.
While laws are provided to state that giving alcohol to children is explicitly illegal, that does not mean that one could not be charged with contributing to the delinquency of a minor. The reason why it is separately stated, is because giving alcohol to a minor is a more specifically "heinous" crime, and thus treated with a larger fine, or longer jail time.
Drug paraphernalia was made illegal in order to be able to convict drug addicts of something even if they no longer have actual drugs on themselves. Drug paraphernalia used to be simply evidence of drug use... then they turned it into something illegal itself.
The "contributing to the delinquency of a minor" is a specific catch all that holds that if you are a contributing factor in that child breaking the law, then you are at fault yourself. In fact, giving children alcohol is typically charged with not only providing alcohol to children, but with contributing to the delinquency of a minor.
You're showing a profound lack of understand of the law in this case. The DA is directly stating that any teacher who materially contributes to a child of age 16 or under having sex could be arrested for contributing to the delinquency of a minor. AND HE IS MAKING A VALID LEGAL ARGUMENT.