a user edit that removes "update" permission fails

I'm working on a new module: http://drupal.org/project/volunteer_timeslots

And I came acoss a perplexing situation: for this module I have a custom node type representing an event is created, and in the node edit form "organizers" are entered, and saved as part of the data in a separate table as the uid values for the corresponding site users.

I set up my implementation of hook_access such that these "organizers" could edit the node where they are listed. The problem: if an organizer edits the node and removes herself from the list of organizers, the update fails because of the code below in node.module which re-checks the permission, even though the edit form should never have been presented without that permission.

It seems this may not present a problem for other modules doing access control, such as TAC, where the node_access table is not update until during the node save. Obviously I could work around this problem, but it seems like this check is unnecesary.

bug is present in 4.7 and HEAD (5.0).

function node_form_submit($form_id, $edit) {
  global $user;

  // Fix up the node when required:
  $node = node_submit($edit);

  // Prepare the node's body:
  if ($node->nid) {
    // Check whether the current user has the proper access rights to
    // perform this operation:
    if (node_access('update', $node)) {
      node_save($node);
      watchdog('content', t('%type: updated %title.', array('%type' => theme('placeholder', t($node->type)), '%title' => theme('placeholder', $node->title))), WATCHDOG_NOTICE, l(t('view'), 'node/'. $node->nid));
      drupal_set_message(t('The %post was updated.', array ('%post' => node_get_name($node))));
    }
  }