It’s very hard to persuade by analogy (because people sometimes find it easier to focus on the differences) but with email you click on an account and expect to see all its emails, until you filter them by unread, flagged etc. Also, your businesslike approach to unread posts on the forum here is a bit like the “zero inbox” approach to emails – it’s probably quite rare with emails, and people often take an even more haphazard approach to forums.
Edit: I realise it’s also difficult to persuade with “people sometimes” and “people often”! I think on Wikipedia they’d add a “Weasel words” tag 