In "To Kill a Mockingbird" what reasons does Atticus give for the children not to play the Boo Radley game?

Atticus saw their play as "putting his life's history on display for the edification of the neighborhood."

He gets mad when he catches them with the note and the fishing pole and Dill with the bell.  He wants them to leave poor Arthur alone.  What he did was his own business and they were to stay away form him.

"If he wanted to come out, he would.  If he wanted to sty inside his own house he had the right to stay inside free from the attentions of inquisitive children, which was a mild term for the likes of us."

They were making a scene in front of the whole neighborhood and it embarrassed him.  He wanted the kids to learn what was right and what was acceptable.  This begins some of the lessons he teaches his children along the way.