genuine question, because i am confused about this: how does this work on mailing lists or quotes in messages?

it doesn't make sense to me that reddit has to remove messages when mailinglists don't because they can't. on some systems quotes are implemented as embedded links to the original message, so if the message disappears, the quote disappears. on hackernews quotes are copies that can't be removed.

why should personal rights go so far as to remove your contributions to a public discussion?

shouldn't that be limited to actually removing only the personally identifiable information that is embedded in the messages?