I have had that very discussion here on FR. Look at something as simple as a traffic citation. If you go to court and say you are not guilty, then the cop says you are, what happens? You are guilty
What if you had someone riding with you and they say you did not commit the violation? You are guilty.
You are always guilty, unless you can conclusively prove that you did not commit the violation (video, proved physical presence somewhere else etc)
In Traffic Court, or district court, the word of a police officer is taken as unquestionable truth. This is in all effect — a presumption of guilt.
The cops word is held to be true, your words if you speak them are considered doubtful. And if you do not testify, what saves you from his accusation? It is exactly a presumption of guilt!