Why does Atticus tell Jem and Scout they have to change their snowman?

Atticus thinks Jem should change the snowman because it too closely resembles their neighbor, Mr. Avery, and not in a very complimentary way either. "Using bits of wood for eyes, nose, mouth, and buttons, Jem succeeded in making Mr. Avery look cross" is the way Scout describes Jem's creation.  Even Miss Maudie is a bit appalled, encouraging Jem and Scout to modify the snowman so as not to hurt anyone's feelings. Because Mr. Avery "is sort of shaped like a snowman," his feelings would be hurt with this caricature.