Ability to set start value of the hierarchical select

If the the hierarchical select dropdown is set to 'enforce deepest', on selecting the top dropdown category, the other dropdowns will appear automatically with default values. However if you don't hit 'add' and the term is not added to the dropbox, the term is still saved for the node when the form is submitted.

The select dropdown is set dynamically using the form_alter hook like so:

$form['taxonomy'][20]['#title'] = t('Business listing categories');
$form['taxonomy'][20]['#description'] = t('You must select at least one category.');
$form['taxonomy'][20]['#required'] = 1;
$form['taxonomy'][20]['#weight'] = 5;
$form['taxonomy'][20]['#type'] = 'hierarchical_select';
$form['taxonomy'][20]['#hierarchical_select_settings'] = array(
'module' => 'taxonomy',
'save_lineage' => FALSE,
'enforce_deepest' => TRUE,
'params' => array(
'vid' => 20,
),
'animation_delay' => 400,
'dropbox_title' => t('These are the terms you selected'). ':',
'dropbox_limit' => 3,
);

Is this a bug? Is there anyway around this?

Thanks for any help :)