Designed behavior, as a global announcement is in every forum, allowing replies can't really be refined to a basic set of permissions.
Technically one could uncomment the check in addpost.inc.php, but it's completely unsupported; but you'd be looking at:
if ($issticky == "2" && ($user['USER_MEMBERSHIP_LEVEL'] != "Administrator")) {
$html->not_right($ubbt_lang['NO_R_ANNOUNCE']);
}
Though, that'd make it more of a global sticky post