The jury is instructed as such by the judge.

 Quote:
3.7 PLEA OF NOT GUILTY; REASONABLE DOUBT;
AND BURDEN OF PROOF

The defendant has entered a plea of not guilty. This means you must presume or believe the defendant is innocent. The presumption stays with the defendant as to each material allegation in the [information] [indictment] through each stage of the trial unless it has been overcome by the evidence to the exclusion of and beyond a reasonable doubt.
...

http://www.floridasupremecourt.org/jury_instructions/chapters/chapter3/p1c3s3.7.rtf


Fair play!