The way I see it is the polygraph is just as imperfect as all other parts of the hiring process. Yes someone can fail a poly even though they were telling the truth, but they can also flunk the background investigation because of a disgruntled former employer or ex-boy/girlfriend, or because they missed passing the written by one question by filling in the bubble wrong or not erasing enough. Or they may have tripped on their shoelaces on the track and failed the 1.5 mile run by 10 seconds.
None of the steps are perfect and qualified people get knocked out all the time for trivial reasons. Sure failing a polygraph may brand DQ you but its not like you are convicted of a felony and can't work anywhere. Other agencies will do a new polygraph and you can get a fresh start.
"Ray, when someone asks you if you're a god, you say YES!"