auto-follow a discussion when you've posted to it
Using Jamroom
What is happening on your system with that user is that at some point they were in a quota that allowed a forum on their profile. During that period they set the notification 'Forum Updated' to 'off' then were changed into a quota that is not allowed a forum on their profile. The result is they are stuck with the 'Forum Updated' setting set to OFF.
In the next version of jrForum 2.2.6 this is fixed. The 'Forum Updated' setting no longer has impact on the watching button. Now only if all notifications are disabled will the notification watcher not be turned on.
. Probably your server company doesn't want to do that for you either so it kind of is left up to you like it or not unfortunately.