bbPress uses a different model of work with topics and replies privileges comparing to the WordPress posts. While bbPress adds ‘delete_topics’, ‘delete_replies’ it really uses just ‘delete_others_topics’ (includes/topics/capabilities.php, line #180) and ‘delete_others_replies’ (includes/replies/capabilities.php, line #162). So default bbPress permissions model does not allow to a user to delete his own content. User should have a ‘moderate’ or ‘delete_others_%’ capability to get access to delete operation. But with this access user can delete a content from any user/author.