There are smarter people around here than me on this topic, but I think that because you run the u.activate(); command, they’re already fully activated and the only thing they’re doing through the email is changing their password from the initial random string that nobody knows.
There is a notion that inactive users eventually stop getting emails, but I’m not sure if that would come into play here for “auto-watched” categories, I have only really thought of it in terms of the digest/summary recipients. Hopefully someone else can chime on that answer.