Although the permission to delete discussions is granted only to mods and admins, normal users are able to delete their own discussions as long as nobody else has posted in them. I assume this to be a feature, not a bug.
This however includes the ability to undelete their own discussions even in the case when a discussion has been deleted by a mod or admin. To prevent this the mod or admin needs to post something in the deleted discussion, either prior to or after the deletion.
I guess, most mods and admins are not aware of this necessity.
