I just want to underscore what Nachalink123 has said about not knowing until it happens to you.
Even then, for a long time I thought it was only happening to me on the DNA boards, so I struggled on my own to figure out which rules I was breaking. I'd noticed that some conversations seemed to be a bit odd, but I figured it was the people who were odd. Only later, when I learned through private messages about chunks of the conversation that had been wordlessly deleted, did certain comments and attitudes make sense from other customers.
I knew things were truly a mess when I would go to post something and have to ask myself, "I'm sure I already said this, but was it in a deleted thread?"
This secretive approach to deletion is out of step with what people have come to expect from message board culture over the past couple of decades. Not only does it leave the customer questioning him/herself, but it has them questioning others. It also suggests that the admin isn't confident enough in his/her reasons for deletion to state them and possibly incur further discussion.
Volunteers with limited time are not an excuse for sub-standard moderation. As I've said before (unless it was deleted), Ancestry can't be advertising on prime time television then shrugging helplessly because they rely on volunteers to moderate the forums. Not if they still expect me to be a satisfied customer.
(I understand the Rootsweb holdover weirdness, but as a customer, I shouldn't have to know about that to make sense of things. I should see only timely, professional conduct geared toward keeping the customer base happy and well informed.)
Also, RWAdmin mentioned (with some exasperation) that replies to a deleted post within a thread will also be deleted. Perhaps RWAdmin isn't aware that some users use the "flat view" option instead of "threaded view." This means that every post appears to be at the same level - all a bunch of replies to the original subject line. When replying in flat view, there can be a tendency to just click the last "Reply" at the bottom of the screen - people don't always think of it as a reply to one specific post.
So, I would suggest that RWAdmin keep this in mind when becoming frustrated that people don't understand why child posts are being deleted with the parent. For some people, all they know is that they replied to one long thread, and now their post is gone. Ideally, Ancestry would change the flat view to a nested view hybrid where replies in flat view are indented.
Until then, admins should be aware that flat view is a specific feature offered by Ancestry, and therefore deleting replies requires more explanation than the admin might like. I would suggest something like "This post was deleted because it was a response to a post that no longer exists." (The deletion message for the parent post should show up on the forum where the original post used to be, with a stated reason for deletion, but it doesn't seem that Ancestry is set up for that.)
Yes, this is all more time consuming than just whacking away the allegedly bad posts, but it's what happens on well-regulated forums. If you can't take the time to do it right, don't volunteer. If Ancestry can't find volunteers willing to spend the time, then I guess they need to start paying up.