Why is it hard to understand? Because your average user will not "wireshark their own cables", and those who do will use different methods of secure communication. You'd be surprised how many users are unable to bypass various extremely simple restrictions - and this is the target market for these features.
On top of that self deleting email will make it also a bit harder to prove that the screenshot is real - unless you get a court order to get the senders outbox folder from google and the data still exists there.