In short, POPFile users would see little benefit from the addition of this feature. Most spam messages today come with forged From: and Reply-To: headers. Bouncing a message would thus lead to bounces in the inbox of somebody completely innnocent, while the spammer will never know. Here is the story of someone whose email address was used by a spammer: My Short Life As An Unintentional Spammer.
Bouncing may have had its value when spammers still used valid from and reply-to addressses. Today, spammers really don't care whether they send their messages to existing accounts or not. They just pump out as many messages as possible and hope that somewhere someone is dumb enough to click and buy.