That's the only way a thread is considered "subscribed" -- email notifications is part of it, being tracked in the UserCP is another -- and there's several ways to be subscribed to a thread.
You can set the option to automagically be subscribed to every post to which you repond (in UserCP -> EditOptions -> Use 'Email Notification' by default?
You can check the "Email Notification" box when you are replying to a thread.
You can click on a link at the bottom of the thread, "Receive updates to this thread. "
To unsubscribe, you have to manually unsubscribe from each thread from within the UserCP -> VBhome menu.
Granted, it's not a very good way to casually watch threads that you're only vaguely interested in.
-jon-