So we could flip the management of the list on its head, then, and make the list wide open but blacklist spammers ... except that you then find yourself in a reactive mode. In other words, spam gets onto the list because the list is open, so you add the sender's email address to the blacklist, which works until they pick another email address and you are waiting to spot spam again. That *potentially* is a tougher approach to take because if someone isn't *actively* blacklisting the spammers, you could end up with a lot of spam on the list once someone finds it.
This may still be a better way to go. I'm not arguing either side :-). I'm just highlighting a potential drawback to going the "open" route.