Exactly. Polygraphs (and voice stress analysis, and fMRI, and any other so-called deception detecting technology) work because they frighten people into fessing up. THAT'S why police departments still use them. The science they are based upon is sketchy or misleading at best, and completely dismisses the inherent complexity of an act such as lying.
Except that (and this has been repeated time and time again above) -- it will NOT devour 'every bit of matter between the black hole and the center of the earth'. The chances of it hitting anything, even if it passes through the nucleus of an atom (which, relative to the size of the black hole, is largely empty space), is minuscule. The black hole would be so tiny, and its gravitational pull so slight, that the chances of it sweeping up any matter at all (let alone the entire planet) before it evaporated are not even worth bothering with.