why does atticus choose not to reveal to his children that judge taylor appointed him to toms case? how does his decision affect scouts perception of her father?

In chapter 16, Scout overhears men talking about how Atticus came to defend Tom Robinson,

"you know the court appointed him to defend this nigger...Yea, but Atticus aims to defend him. That's what I don't like about it"

I think Atticus wanted his children to think defending Tom was a choice he freely made: standing up for the right thing is important. I think, however, that Atticus always acts according to his conscience.