Doesn't mean they don't work as an intimidation tactic. Though most of the agencies that use them are probably filtering out a lot of good employees by putting them through a test designed to mess with them. Instead of focusing on whether the person has the skills needed for the job.
People need jobs and are willing to put up with lots. Going through with a polygraph is likely just one of a number of the things that people in these industries have to put up with.