Only just read this thread. Added a ticket to the tracker to add the check box to manual authentication.
As mentioned in one of the replies, a suggestion was made to resolve this by commenting out a line in the php file.
While this works, it will have a negative impact on upgrade paths as you need to append the code each time. The file to edit at the moment is editadvanced_form.php (line 222 in moodle 2.6 to be exact).