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:
Code
			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

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