The options aren't "innocent until proven guilty" or "guilty until proven innocent" - there's also "what is the evidence that I've seen telling me is most likely so far?" Actions based on this should be heavily checked by recognition of your uncertainty, but if you are wanting to come to the correct answer most often, you'll use something closer to that. The purpose of the justice system is not "to come to the correct answer most often", but to determine action (typically severe action) and so needs that check.