We reverted that change because it brought confusion. So the checkbox will be disabled by default from now on.
JBWolverine wrote
If I understand the situation correctly, the default of having the "Alert me by email when someone replies to my message" box checked as the general setting is to allow non-members of a forum to follow posts on the forum.
This makes sense if the forum is PUBLIC. However, it should not apply if the forum is PRIVATE.
PRIVATE forum members should have an personal settings override for the "Alert me by email when someone replies to my message" box so that they can turn it off for all of their posts, if desired.
Sorry, but you misunderstood it. That checkbox allows any user to receive new posts in the related topic by email. So if the user was allowed to post, then he/she has read and write permissions on that forum. In other words, it does not matter if the forum is public or private because the user already has access to it. If the administrator decides to ban a user that had such checkbox checked, that user will not receive new posts by email.