policy.drush.inc file in ~/.drush folder is not automatically included

My policy.drush.inc file in my user's .drush folder:

function drush_policy_sql_sync_validate($source = NULL, $destination = NULL) {
  return drush_set_error(dt('Not allowed'));
}

Output when not explicitly including .drush (see attached for full debug output):

vagrant@ubuntu-10:/srv/www$ drush sql-sync @prod @local --no-cache

You will destroy data from drupal and replace with data from test.org/testorg_prod.

You might want to make a backup first, using the sql-dump command.

Do you really want to continue? (y/n): n
Aborting.                                                                            [cancel]

Output when explicitly including .drush (see attached for full debug output):

vagrant@ubuntu-10:/srv/www$ drush --include=~/.drush sql-sync @prod @local --no-cache
Not allowed                                                                          [error]