Firstly, who has more reason to research lie detectors and realise they don't work, a criminal or an innocent person? Clearly, it's the former, so you're using

Firstly, who has more reason to research lie detectors and realise they don't work, a criminal or an innocent person? Clearly, it's the former, so you're using a "trick" that is weighted against innocent people. Secondly, most criminal justice systems reward early confession with reduced sentences and plea bargaining. In such systems, presenting a suspect with fabricated evidence—which is what a "positive" lie detection report is—seems just as likely to induce a false confession as a real one. If we allowed cops to plant evidence and falsify statements, we could probably get a lot more confessions. If we eliminated the "innocent" verdict at trial, or reintroduced trial by ordeal, we could do the same. But we don't do this because these are fundamentally unjust, and so is allowing law enforcement to claim they know people are guilty due to magic.