alt Hacker NewsIf PIs can "legally" do it then it sounds like there is a law which allows them to do it. That law can be revoked (unless the power comes from Constitution which would make it effectively impossible to revoke).
Note that PIs are effectively illegal under GDPR by default. They would generally need to provide Article 13 notice, i.e. you would become aware of them unless they were just asking around without actually following you. Member states can make them legal though (via Article 23) and likely in many cases they have done so.
In the US, PI licensing is only about PIing for hire. The actual act of going through public records, following cars and whatnot do not require a license, you can spy on anyone without a license as long as you don't get paid for it.
EU is more complicated, but Article 14.5.b allows withholding notice if it would impair/defeat the purpose of processing. The PI must however apply "safeguards", whatever it could mean.