Annoying for sure, but one benefit of this approach is due to forwarding. For the moment, let's assume you have a newsletter you enjoy. If you forward an instance of that newsletter to your friend, and they aren't expecting it, they might hit unsubscribe. With one click, they'll prevent you from receiving future newsletters.
I think the better approach is simply showing the "Intended for johndoe@example.com" next to Unsubscribe, but I could see why they ask for your email.
as per RFC-2369 and use the same mailto link inside the mail body. This is convenient, conventional and solves the problem you describe, while also allowing users to add feedback as they see fit in the body.
I don't see why you have to involve the web at all, but I can tell you that if I have to go through a bunch of bullshit when I want to unsubscribe I'll just mark it as spam instead. However appreciated and anticipated your newsletter is, you have to consider that most newsletter subscriptions are probably either accidental (failed to uncheck some box when signing up for something entirely different) or straight up unsolicited, and people like me will basically purge all their subscriptions without discrimination regularly as the crap builds up.
> I don't see why you have to involve the web at all
Because List-Unsubscribe is very new and not supported by all email clients or ISPs (inbox service providers). Since adoption is so spotty, most ESPs (email service providers) start from a baseline of a web unsubscribe system and (maybe) supplement that with List-Unsubscribe as well.
RFC2369 from 1998 is "very new"? Funny. I guess it's either a matter of perspective.
Adoption of it doesn't matter insofar that you can use the same link in the body as a fallback, which was part of my suggestion.
"ESPs" don't care because cumbersome and convoluted multi-step unsubscribe is a selling point for the majority of people who use such systems. They have moved from simple mail based unsubscribe which was the norm for years.
FWIW, this solution isn't as easy to implement as you'd think. I've seen unsubscribe pages harvested for email addresses when they show the full address and used urls/tokens that weren't sufficiently secure. In the case I'm thinking of, the home-rolled algo that generated the unique links was bugged enough that you could reverse it, and I was surprised that someone actually took the time to do so.
I think the better approach is simply showing the "Intended for johndoe@example.com" next to Unsubscribe, but I could see why they ask for your email.