I don't really see any danger since admins are not responsible of criminal acts (is it actually "criminal" or "reprehensible"?) committed by some members in real life. The messages are posted on the forum, but not stolen in the forum, they are stolen in real life, and there's nothing can prove to the admin or anyone that these messages have actually been stolen, and not created by the poster's imagination.
Anyway, I agree that a forum should not be the place for this kind of services. When it's time to time, to help a member who is active and often participate to the forum's life, I don't see any problem. But when people register to this forum with the unique motivation is to get translation help, I think it should be moderated.
@Texan : What is the point in trying to have complex talks and relationship with someone you don't even have 1 language in common for basic communication? It means you always need an intermediate to assist you, and will have no private life at all with this person... weird.
The email you ask people here to translate are private materials, I would not let hundreds of unknown people read my private correspondence with a girl I like. Maybe a close friend, at a pinch! Not even sure I would...