Never said it was but due process is. While there is no 'presumption of innocence' spelled out per se, in the 6th amendment, the law does require that the state prove guilt beyond a reasonable doubt and until such a time that they do so the law requires that the defendent be presumed innocent. It is a legal right of anyone accused of a crime in the United States of America.
Its nice that you 'know' these things, but it doesnt matter what you or anyone else 'knows', what matters in our system of jurisprudence is whether the state can prove it beyond a reasonable doubt. For example, in the case Florida v Casey Anthony, the state failed miserably in that regard and as a result the jury acquitted the defendant in short order, and rightfully so.
I recall a lot of people 'knowing' the earth was flat at one time and that it was the center of the universe too so its a good thing that we dont depend on that when it comes to taking away people's freedom and liberty otherwise there would be a lot more innocent people locked up in our prisons than there already is.
I would guess that the state will be more successful in the prosecution of Jeffs especially since he has now chosen to represent himself, and if the evidence shows he did what he is accused of he certainly should be held accountable for that.