It's pretty common to require sign in to change email preferences. Not out of malice, but more that if you want to have more than a global unsubscribe, you then have to allow users to see the prefs for a given email address without being logged in. It gets tricky quickly.

I'd love to see a blog post about best practices when you have a few different options for in your email prefs & you want to avoid people having to log in.