while I'm sure plenty of people in law enforcement haven't 'gotten the memo' that polygraphs don't actually work, I suspect in many cases they do them anyways exactly for the reason you describe.

Reminds me of one of my favorite scenes from The Wire, Bunk using his bag of tricks to get someone to talk. Whole thing is hilarious, but the lie detector stuff starts here:

https://youtu.be/rN7pkFNEg5c?t=92