Today there was hella confusion in my group until I figured out what happened through the logs. A member deleted a message, which apparently was the whole topic, resulting in the deletion of the topic, and then resent it. People reading via email received duplicates, and people reading via email and web were massively confused.
Allowing people to delete and resend messages creates a loophole around the disabling of editing messages, which my group has done. Instead of editing a message, which members can no longer do, enterprising, creative members have figured out that they can simply "delete" (I use quotes because, of course, the message is never deleted from emails) their message and resend it. This has the EXACT SAME EFFECT as editing a message, which my group - as well as this very beta group - has disabled for good reasons, including the email-archive mismatch and resulting confusion, and the receiving of essentially duplicate emails by group members reading by email.
I understand from, prior conversations here about it, that some think that legally, members are allowed to delete their messages. IANAL but I disagree. Users grant to groups.io, via the TOU, a "perpetual and irrevocable" right to publish their content (quoted below). "Irrevocable" would seem to mean that users cannot revoke from groups.io the right to publish their content; i.e., they cannot unilaterally delete it.
So I think that deletion should be disabled for the same reasons editing can be disabled (email/archive mismatch and duplicate messages, possibly even a flurry of them, as has happened in my group at various points when overzealous members keep trying to "clarify" or, in essence, edit their posts). The confusion is worse when a member deletes a whole topic, which it seems they can do when their message is the only one in it.
By uploading any User Content you hereby grant and will grant Groups.io and its affiliated companies a nonexclusive, worldwide, royalty free, fully paid up, transferable, sub licensable, perpetual, irrevocable license to copy, display, upload, perform, distribute, store, modify and otherwise use your User Content in connection with the operation of the Service.Emphasis added.
Messages are the sole opinion of the author, especially the fishy ones.