During political debates on TV: why aren't candidates connected to lie detectors with results displayed live?

I think a scroll at the bottom of the screen should display whether or not the candidate is telling the truth or not. Agreed: lie detectors aren't 100% accurate. But as a viewer and voter I'd like a 2nd opinion! How is a lie detector machine which produces "possible" inaccurate results any different than the opinionators on CNN, FOX, or MSNBC following the program? I'd prefer the lie detector. And I'd love to see the detector hooked up to a stun-gun which zaps them every time they lie! Wouldn't you?