It's a bit of a joke, really. I doubt there's anyone reading this on here who didn't already know polygraphs don't work. The only people who don't seem to know polygraphs don't work are the ones making national security decisions relating to polygraphs.
My guess is that this works just like how it does in corporate security: It is vastly easier to add a new security policy than it is to remove it. Even when everyone knows something is useless or perhaps even harmful, nobody wants to be in a position to be held culpable on the off chance that retiring the policy is implicated in a future breach.
Polygraphs are a $2 billion industry, and Upton Sinclair said it best: "It is difficult to get a man to understand something, when his salary depends on his not understanding it."