Not sure if issue on views or d.o side, but operator is like one level too deep https://api.drupal.org/api/views/includes!handlers.inc/class/views_join/7

screenshot: http://gyazo.com/2777df780d485ad996a7c671232f76c2

 *   - - field: Field or formula
 *       in formulas we can reference the right table by using %alias
 *       @see SelectQueryInterface::addJoin()
 *   - - operator: defaults to =
 *   - - value: Must be set. If an array, operator will be defaulted to IN.