So, fireworks used by the defendants may have started the fire. And its not important for the jury to know this... why again?

The focus has become whether or not the defendants' burn pile started the fire. If defense can raise a reasonable doubt about the cause (being their burn pile) then they will get off. So assuming their illegal use of fireworks was actually the cause, then prosecution is pushing a losing case and they could be found "not guilty", even though they in fact started the fire .... going to be a lot of pissed off people if that happens because of this!