Otherwise if you don't hold a clearance, I don't think they can do anything to you (however, I am not a lawyer so don't take this as a legal advice etc etc...)
I remember reading people with clearance were not allowed to access wikileaks, but I can remember any follow up "X got fired|trialed because accessing wikileaks". Not that I think it can't happen if we didn't know about it, of course.
Of course it's also a great medium to screw someone over, but that's a whole other thing.
It effectively lets through psychopaths, very good liar or those that know how polygraphs work and presumably practiced passing it.
They are typically used in criminal investigations as a way to make suspects feel uneasy so investigators can pick up visual tells.
The fact that police services can use it to extract confessions is just a nice bonus, and it's really a really cheap alternative to conducting a complete and thorough investigation. Police policy has for a long time favored confessions over investigations.
It is, however, still a part of getting or maintaining clearance. A friend of mine wound up changing jobs simply because he failed the polygraph twice and didn't want to bother going in for a third.
Utter bollocks, them.