On many sites (that have more than a simple newsletter), the email preferences are part of the account settings where you do things like change your password, a

On many sites (that have more than a simple newsletter), the email preferences are part of the account settings where you do things like change your password, avatar, possibly manage your billing information and shipping address, etc. You wouldn't want any random anonymous person to be able to access that info, so of course it requires logging in by default. While those systems should ideally have one-click unsubscribe functionality nowadays, there are probably quite a few that don't.