Thats the law of proof - if there is an element of doubt they get away with it. I have had so many things happen to and around me over the years you do look at the law as a joke, I have heard many a senior policeman say you need 10 written sworn statements by priests and bishops to get a conviction nowadays (although even their integrity is often in doubt!). I got assaulted by a guy on the London underground, nothing serious he looked out of it, just shoved me in the chest for no reason, I held my nerve and just got off. Called the police afterwards and they wanted me to make a statement at a police station, I asked what would get done, and really apart from adding a 1 to the statistics nothing. I find law frustrating as it is hardly ever possible to implement it!
Anyway I digress!
Bit different if it goes down the civil route - the proof doesn't have to be 100%. 51 to 49 is enough in a civil case.