Why are policemen always presumed innocent before any trial?

Time and again we see trial by media first then the inevitable not guilty verdicts that follow extensive brainwashing techniques by the press and inaccurate and misleading 'reports'.Jurors are always asked by the trial judge...'to dismiss from your mind anything you have read or seen on tv or newspapers lol and ths's possible!This cannot be fair to the victims of police killings by car or gun or police custody deaths surely? can it?