“Teaching about the flaws of polygraph testing is not inherently illegal. […] Dixon was charged after he helped undercover agents learn to cheat the test after they told him specifically that they intended to lie as they applied for federal jobs.”
So Dixon’s charge isn’t simply helping people beat a government test. It’s aiding candidates in gaining government jobs, even though he knows that those candidates are unqualified for those jobs (according to official measures). I think that makes Dixon’s charging more understandable.
Now, one may disagree that the questions they ask in such tests are an accurate way of finding good candidates. I don’t know what sorts of questions they ask. But the government’s response is understable, because they believe that the tests really do qualify one for the job.