If "View active topics" is set to "No", as it is by default, and you go into a forum's settings and change the radio button to "Yes" and Submit, you get this information message:

Information

Forum information updated successfully.

Now you are able to set permissions for this forum.

This implies (and I know it does not state it explicitly) that the changing of that setting somehow requires the forum permissions to be reset, or that making this change has somehow enabled the ability to set permissions, which is not the case. Without testing, I imagine the same message gets displayed when other, nonpermission-related changes are made.