Why can't Jem speak about the trial?

Hey I just finished reading that book for school. So anyways he gets mad when he talks about the trial because he was there and he witnessed everything firsthand. He saw how Tom Robinson couldn't have done it because his left hand was injured during a tractor accident and he no longer has use of it. Mayella was savagely beaten with someone who dominantly used his left hand, like her father Bob Ewell. Atticus shows that Tom Robinson couldn't have done it because he doesn't have use of his left hand. Although he makes that quite clear, all of the jury is too prejudiced (remember they are in Maycomb County, Alabama in the 1930's) to acquit him so when they come out and announce their verdict of guilty, Jem starts to become angry and emotional about it because he doesn't understand that although the right thing to do is acquit Tom Robinson because he is innocent, that won't happen because the adults are, as I said, too prejudiced. Afterwards, Jem struggles to understand how this could happen and justice was not issued, so he begins to ask Atticus why the courts and juries aren't always fair. Atticus then says that if only all the adults were like Jem, instead of just a few, then Tom would be free. ---- Anyway hope this helps! I know I wrote a lot sorry about that.