Thanks for the captures.
Subscribing to a forum depends on different settings with-in Moodle. http://docs.moodle.org/20/en/Forum_settings#Subscription_mode
One option is shown at the moment when replying to the forum, as you captured in this reply, it allows you to send Email copies of the selected forum.
Another option can be selected from your Moodle user profile, as you are able to select Auto-subscribe and forum tracking.
Another option depends on your Instructor settings for the particular “forum”. Therefore, if the instructor has set the forum to “Forced subscription” you are going to be receiving everyone’s posts and comments.
I would suggest contacting each instructor and request permissions to handle your own subscriptions.
Hope this helps!