What is humorous about Tom's brie membership in the Cadets of Temperance?

This is a group that swears to abstain from smoking, drinking, swearing and other various social vices. Tom's membership is humorous because he is a kid who did these things; they were a part of his personality. With Tom all these behaviours didn't seem so bad because he was such a likable kid. Tom wanted to join mostly because he liked their fancy outfits.