A fortress has a wall on the edge of its roof, called a parapet. Archers use the parapet for protection as they shoot at attackers. In the current emotional circumstances of the oil spill, the Obama administration is working on looking tough ("macho talk" from your previous question) by attacking business. Your article suggests that businessmen fear if they publicly criticize the Obama administration ("stick their heads above the parapet") they might be the next one Obama attacks.