The typical citizen is obliged to be honest and upstanding, and has a corresponding right to be presumed to be so. So in law, a person is deemed to be innocent until it is proven beyond reasonable doubt that he or she is not.
So if you wish to accuse someone of a crime, it is up to you to prove your case beyond reasonable doubt. This is not always easy, and a lack of evidence or a lack of witnesses (or other things) can make it impossible.
Defending yourself against a criminal charge is stressful; a lot is at stake, and mistakes can be made. Accusing a person of a crime whilst knowing the accusation to be false, and lying in court are both frowned upon. In France, an accusation known to be false is a "dénonciation calomnieuse" and making one is a crime.
It doesn't take a brain the size of a planet, nor a Masters' Degree in logic to realise that failure to prove your case beyond reasonable doubt against a person who is presumed to be innocent, does not automatically mean that your accusation was false, or known to be false. And yet this illogic was written into French law: if you failed to prove your case, you were automatically guilty of "dénonciation calomnieuse".
It took a ruling by the European Court of Human Rights on 30 June this year to correct this error on the basis that it overturned the presumption of innocence. I think that the ECHR is a Good Thing.