It is because we follow these cases from day one, all through the searches and the investigation, the arrests and then follow the trial.
Members of the jury don't do that, and/or if they do and are honest with the court, they will be excluded from the jury.
There is no rule here that says "innocent until proven guilty." That's for a court of law. When we watch the story unfold, read everything, sleuth everybody, etc., etc. making judgment calls is to be expected.
Hope that helps,
Salem
PS - not to mention the evidence that has been presented. For me, I can think of absolutely no reason for Tori's blood to be mixed with sperm. I'm not buying that he dripped at some earlier time and left it on the door molding - nor am I buying that the blood in the bag mixed w/MR's blood was put there by TLM. Just not happening for me.