Drupal 8 field API vs. $form_state
Taken from http://www.drupal.org/node/2350819:

If you import a field, it automatically goes into the disabled section because the fact that a field is shown or not is now stored in the entity display config entities, which fields do not now anything off. I should check what our default behavior is, but this is probably by design - which makes sense actually. And if not, it'll be a feature request.