About the US Government requiring a polygraph to hold a Top Secret clearance and higher....
That's both true and false.
If the person is in the military, then a polygraph isn't taken. The reason is that duty assignments are considered "involuntary" and as such requiring a polygraph would be unjust. However, civilian employees are required to pass a polygraph. In fact, there have been a few cases where a military member left the service and immediately came back to the same organization as a civilian and was required to take a polygraph .... and failed ... so were denied employment at the very same organization that they just left.
I used to be active duty military and at the end of my career was working in WHCA (White House Communications Agency). That position required a TS/SCI Yankee White investigation (they tend to be a bit paranoid when you're in a position where you could physically touch the president without the Secret Service batting an eye). It was an interesting assignment, but eventually I left the service and took employment with a government contractor and was required to obtain another TS/SCI clearance. And yes, a polygraph was required.
The first polygraph that I took had "issues" and I was rescheduled to take a second polygraph. I too had issues with the polygraph since it felt to me that a game was being played where I wasn't informed as to the rules. So in my typical fashion, I researched polygraph technology and found out quite a few interesting things. One item I encountered was a reference to a classified study on polygraphy. I wasn't able to obtain the study itself, but assuming that the study reflected the publicly available information on the polygraph and if I were to be a classification authority, then I too would have classified the study.
Because simply, the publicly available information boils down to this.
Polygraphy as a means of detecting true or falsehood, it's totally worthless. But as a means of eliciting voluntary confessions from naive subjects, it is extremely effective.
Notice the phrase "naive subjects."
Let's just say that on my followup polygraph, I understood the rules of the game, informed the polygrapher and she was the one who had an unhappy time. The results were inconclusive and I did get my TS/SCI clearance.
Yes, as a civilian employee in the United States, you are required to take a polygraph to obtain a TS or higher clearance, but that it just one element of the clearance process and it doesn't require the subject to actually believe in the effectiveness of the polygraph.