“Side A” here sounds an awful lot like a “Grand Jury.” There, the defense does not have a say, because the defendant isn’t being charged. Rather, the Jury is saying whether or not the prosecution can even go to trial. So while the jury can evaluate the credibility of a witness, the usual question is whether or not the prosecution has enough evidence to even go to trial.
This may sound unfair, but it’s a brake on the prosecution. The defendant still gets his trial and all, but this is an added hurdle for the prosecution.