Members have discovered locking and moderation subversion by deleting a post. I realize (I think?) that your position is that members have some sort of legal right to delete their posts, but I'm not sure about that. (They can't delete posts in some other forums, notably the news outlets and craigslist, and some others I know of.) Their emails are still around and are not being deleted. I would prefer it if in a group where editing posts is disabled, deletion could also be disabled. There's not much difference, to my mind. If we/you can prevent members from editing their copyrighted creations, we should be able to stop them from deleting them as well.
I didn't mind this lack so much until I became acutely aware of the hole created by deletion, through which both moderation and locking can be undone. And it's precisely in a locked or moderated thread where someone may wish to delete their post.
I hope you will give this further consideration.
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I wish I could shut up, but I can't, and I won't. - Desmond Tutu