How does Atticus explain his decision to defend Tom Robinson?

Atticus did not tell Scout that he hadn't volunteered to defend Tom Robinson. Instead, he said exactly what he felt at the time. Atticus believed that everyone had the right to representation, even when it was a losing battle, and even when it wasn't the most popular thing to do. He did the right thing, and that was what he wanted the children to understand.

"I couldn't represent this county in the legislature, I couldn't even tell you or Jem not to do something again... I could never ask you to mind me again... every lawyer gets at least one case in his lifetime that affects him personally. This one's mine, I guess."

Atticus explains to the children that he was doing the right thing and that Tom should have been treated like anyone else.