If the form disabled, or one action button is disabled, grouping actions to dropbutton is clearing current disabled state and form become submittable.
Steps to reproduce
1) Create some content type
2) create content of this type
3) Write a module with hook_form_alter() and alter the some node form with $form['#disabled'] = TRUE;
4) go to the Edit page of created content,
5) see that actions are not disabled and form is submittable.