Actually in our system of justice the plea to a criminal charge is NOT innocent but rather NOT GUILTY.
The difference is that the defendant does NOT have to prove he is innocent but rather the prosecution has to prove he is guilty. If the prosecution does not bear the burden of proof beyond a reasonable doubt the defendant is found NOTGUILTY. It has little to do with innocence and every thing to do with carrying the burden of proof.
Guilty people escape conviction all the time. They are not innocent but merely not guilty.
Have you read the briefs for the defense, interviewed their witnesses, have intimate knowledge of their strategies, cross-examined the evidence, etc...?