In terms of legality there are two separate issues, invasion of privacy and copyright. Things like privacy / libel / slander generally don't apply to the deceased, so in this case I think we can safely ignore that.
With respect to copyright, when you send someone an email you're using an agreed upon protocol (RFC 5322, RFC 5598, etc.) that implicitly grants the recipient a copyright. Now you can say this is a bad argument because no one ever reads those, which is true. But if this weren't true legally, then every time you replied to an email it would be illegal since (in most clients) the full text of the previous email gets copied below the new text. Users also can't really use the platform to violate the copyrights of third parties (e.g. by attaching music or whatever), since the platform is tied to their email account.
From an ethical perspective, the reason we specifically excluded whistleblowers as a user story is that you can't build a sustainable business around it, since you can only be a whistleblower once. we've designed it to force uploaders to have skin in the game. E.g. they can't anonymize themselves, and they're forced to link their Gmail accounts. I think that's enough to prevent the vast majority of bad behavior. E.g. you could also copy and paste someone's private email into a HN post, but it's just not done. The only thing we really do is make the process easier by letting people making redactions, anonymize people, send permission requests, etc., but that's not exactly a selling point for people who would be egregiously abusing the site.