The example here is somebody who apparently needs to take one to access some confidential information - and clearly they take a pass/fail seriously, so it seems some (presumably US?) government agencies do take it seriously enough to make passing one a security requirement.
Good point. A quick Google search doesn't seem to bring up any laws that prevent employers from using polygraph results in determining whether to hire or fire someone.
They are typically used in criminal investigations as a way to make suspects feel uneasy so investigators can pick up visual tells.