custom taxonomy – can't change selections when editing post

Hello,

I have a custom taxonomy (see below for the arguments). When I login as an Author and I edit one of the stories that belongs to the Author, the terms are visibile but the checkboxes are disabled.

But, I have set the “assign_terms” to true and according to the code in wp-admin/includes/meta-boxes.php, they should NOT be disabled since I have assign_terms set to true:

From wp-admin/includes/meta-boxes.php:
$disabled = !current_user_can($taxonomy->cap->assign_terms) ? 'disabled="disabled"' : '';

My Custom Taxonomy args:

, 'rating' => Array(
		'object_type' => Array('fiction')
		, 'args' => Array(
			'labels' => Array(
				'name' => 'Ratings'
				, 'singular_name' => 'Rating'
				, 'add_new_item' => 'Add New Rating'
				, 'new_item_name' => 'New Rating Name'
				, 'edit_item' => 'Edit Rating'
				, 'update_item' => 'Update Rating'
				, 'search_items' => 'Search Ratings'
				, 'popular_items' => 'Popular Ratings'
				, 'all_items' => 'All Ratings'
				, 'parent_item' => 'Parent Rating'
				, 'parent_item_colon' => 'Parent Rating:'
				, 'add_or_remove_items' => 'Add or remove ratings'
				, 'separate_items_with_commas' => 'Separate ratings with commas'
				, 'choose_from_most_used' => 'All Ratings'
			)
			, 'public' => true
			, 'hierarchical' => true
			, 'rewrite' => true
			, 'query_var' => true
			, 'capabilities' => array(
				'manage_terms' => false
				,'edit_terms' => false
				,'delete_terms' => false
				,'assign_terms' => true
				)
		)
		, 'terms' => Array(
			'G' => Array()
			, 'PG' => Array()
			, 'PG-13' => Array()
			, 'R' => Array()
			, 'NC-17' => Array()
		)
	)