It's partially that, it's partially that jurors are not supposed make up their mind till the end of the trial and talking to others tend to make you form opinions.
A judge is expected to be more disciplined when receiving evidence. For example, in a bench trial a judge will hear all evidence including inadmissible evidence. After all it's not inadmissible until the judge rules it so. The judge is then supposed to be able to ignore the inadmissible evidence when reaching his decision.
You are also wrong about the mistrial. The judge was remove mainly because of her remarks on the web site. A new judge ruled that changing judges in the middle was grounds for a mistrial. They then had a second trial where the man was acquitted.
He was acquitted for many reason, the biggest is that it wasn't Harry Potter being kept in the cupboard under the stairs. The kid seems to have had mental problems and a tendency to violence. So the box was the best that his parents could do, and most of the time the door was open.