Here's a patch:

--- a/html/sites/all/modules/community/simplenews_register/simplenews_register.module
+++ b/html/sites/all/modules/community/simplenews_register/simplenews_register.module
@@ -108,6 +108,7 @@ function simplenews_register_user($op, &$edit, &$account, $category = NULL) {
$form['simplenews']['simplenews-'. $term->tid] = array(
'#type' => 'checkbox',
'#title' => t('Subscribe to !newsletter', array('!newsletter' => $term->name)),
+ '#description' => $term->description,
'#default_value' => isset($edit['simplenews-'. $term->tid]) ? $edit['simplenews-'. $term->ti
);
}