The admin setting screen for general backup settings (/admin/settings.php?section=backupgeneralsettings) has selections for default settings and locked settings for options in course backups. If the "Include users" default is set to "off" (unchecked), you can turn it on when you start a course backup on the first page. When you advance to the next page, all user data will be turned off, and locked. If you set the "Include users" default to "on", and leave the setting on when you start a backup, the second page will show you all of the user data selected and not locked.
If the default is "off", you should still be able to backup user data by selecting the option to be on.