After reading src code for contentEditorHelper.configureContentEditorButtons it makes the same assumption that if the publish action 'U' is present then the user can also unpublish aka 'Z'

src/Umbraco.Web.UI.Client/src/views/propertyeditors/listview/listview.controller.js

@@ -79,7 +79,7 @@ function listViewController($rootScope, $scope, $routeParams, $injector, $cookie
                 "canDelete": _.contains(currentUserPermissions, 'D'), //Magic Char = D
                 "canMove": _.contains(currentUserPermissions, 'M'), //Magic Char = M                
                 "canPublish": _.contains(currentUserPermissions, 'U'), //Magic Char = U
-                "canUnpublish": _.contains(currentUserPermissions, 'Z'), //Magic Char = Z 
+                "canUnpublish": _.contains(currentUserPermissions, 'U'), //Magic Char = Z (however UI says it can't be set, so if we can publish 'U' we can unpublish)
                 "rawPermissions": currentUserPermissions
             };