United States Flag Code is actually Federal Law, Chapter One of Title Four of the United States Code, specifically. However, it was written intentionally to be unenforceable. It is assumed that it would be followed out of Patriotism.

There's no opinion about it, this is the way that Our Nation has chosen to be a standard way for us to honor Our Flag. You are free to choose to be an asshole and dishonor it, without the law punishing you, but if I personally see someone disrespecting MY flag, they need to watch their fucking back.