I have a family member who just used a PI to get child support from someone hiding income illegally.

PIs are used to catch cheating spouses, insider trading, theft and corporate espionage.

That doesn't mean PIs or given practices are immune to criticism. However, blunt statements such as "PIs are stalkers for hire" are so sloppy as to be factually incorrect and useless, and comparing them to rapists belays naiveity and an agenda.