If the interviewee's behavior is not indicative of truth then the test serves no purpose other than allowing the interviewer or whoever commissioned the test (like a prosecutor or employer) to invalidly convince other people that the interviewee was lying

That's exactly the point.

That and to convince the interviewee that the interviewer knows they are lying.

Not if you view the interview as a process to expose reasons to not eg hire someone rather than to establish a list of facts or earnest perspectives. Literally just putting the candidate under stress. For this to apply in court you'd have to be suing them for not hiring out of discrimination over a protected class (i think, I am not a lawyer).

I mean maybe there are other civil suits you could file, but I suspect a lot of that would be signed away before the polygraph.

Polygraphs are usually inadmissible in court. It's unfortunate that "usually" applies.

https://www.hg.org/legal-articles/is-a-polygraph-test-admiss...