Setting the value of a single on/off checkbox doesn't work

Confirming this issue. This is also present in 7.x-3.x-dev

Confirmed this issue.

"Unchecked" and "Emptied" option does not work on my boolean field as a checkbox field.

Have you tried manually entering the on value "1" in most cases rather then using the widget defining value?

Confirmed robby's situation.

Faced same issue. I found that state action attached to field wrapper, but not to form element.

In debug console you can see that js action runs on field wrapper - it really sets values for attribute checked.

I couldn't find suitable alter in module. Which allow to change target element.

For my project I use quick tricky script:

(function ($) {

  $(document).bind('state:checked', function (e) {

    // Fix issue with Conditional fields. It applies actions to element wrapper.
    // We re-implement it to form element instead.
    if (e.trigger && $(e.target).hasClass('field-widget-options-onoff')) {
      $(e.target).removeAttr('checked').find('input.form-checkbox').attr('checked', e.value);
    }
  });

})(jQuery);

#6 works for me. Thank you for sharing

#6 also works for me. Thanks a lot.

#6 also works for me.
Thank you, Vitalii T. (vtkachenko).

Add a new javascript file with the code above to your theme folder:
/sites/default/all/themes/[your_theme]/js/[filename].js

Add the following to your theme's info file.
"scripts[] = js/[filename].js"

See guide: https://www.drupal.org/node/756722

Didn't work with jQuery 1.10. Small change, however, fixes it: .prop('checked', e.value);

Also, I'm attaching using js behaviors:

(function($, Drupal, window, document) {
  'use strict';

  Drupal.behaviors.my_module = {

    attach: function(context, settings) {
      $(document).bind('state:checked', function (e) {
        // Fix issue with Conditional fields. It applies actions to element wrapper.
        // We re-implement it to form element instead.
        if (e.trigger && $(e.target).hasClass('field-widget-options-onoff')) {
          $(e.target).removeAttr('checked').find('input.form-checkbox').prop('checked', e.value);
        }
      });
    }
  };

})(jQuery, Drupal, this, this.document);

...and I'm attaching to the form like so:

/**
 * Implements hook_form_BASE_FORM_ID_alter().
 */
function my_module_states_form_node_form_alter(&$form, &$form_state, $form_id) {
  if (!isset($form['#attached']['js'])) {
    $form['#attached']['js'] = array();
  }
  $form['#attached']['js'][] = array(
    'data' => drupal_get_path('module', 'my_module') . '/js/my_module.js',
    'type' => 'file',
  );
}

#10 worked for me, thanks!

Thanks for the report and all the efforts in here. But due to inactivity in this issue for some years and because of the upcoming EOL of Drupal 7, I will close this issue on the way cleaning up the issue queue.

Feel free to re-open as "Needs review" if you found a solution or have a patch to be reported to help others and I will see if we can commit it before Drupal 7 EOL.