They want the jury to make the decision based on the evidence provided by the courts, not the messy accusations and such of the internet. If a jury goes into the case thinking "Yup he did it" due to what they read online, the system has already failed. That's the theory at least.
The idea is that if the evidence of the court of law, and not the court of public opinion, can sway the jury to pass a guilty verdict, then we are less likely to convict an innocent man, and regardless of how horrendous the charges are, or what the public may believe, the jury is supposed to enter with the mindset of "innocent until proven guilty", and without any opinion of the guilt of those being charged until they have heard all the evidence, and the only thing that should influence their decisions is what is said within the court chambers.
The reality is people have WAY too much faith in first accusations and just the fact charges are laid against someone makes people believe that those laying the charges are right, because it is their job to catch criminals, and that alone will sway most juries without much issue.