Why does Scout get angry towards Miss Gates as she teaches them about the horrors Hitler committed and the value of justice and equality?

One day during Current Events, Scout's class gets into a discussion about Hitler and the persecution of the Jews. Her teacher, Miss Gates, speaks at length about how the German dictatorship allows for the Jews to be persecuted by a prejudiced leader, but she claims that in America, "we don't believe in persecuting anybody." Scout finds Miss Gates hypocritical because she remembers that on the day of Tom's trial, she overheard Miss Gates say that she thought it was, "time somebody taught them a lesson, they thought they was getting' way above themselves, an' the next thing they think they can do is marry us." "Them" meant black people. In Scout's mind, this doesn't make sense and she goes to talk to Jem about it. Jem responds very angrily, and tells her he never wants to talk about anything having to do with that trial again. Scout is taken aback and goes to Atticus, who assures her that Jem just needs some time to think about things, and then he'll be himself again.