@pfvspnf
From a regulatory perspective there is no difference between a P2F pilot and a regular pilot. Both must be qualified by whatever flight authority to serve as FO / SIC for the flight. It's just one is willing to pay and the other wants to be paid.
Some P2Fs are non-scheduled cargo ops (i.e., FAA Part 135 in the US) -- these flights don't even require an FO as they may be operated with a single pilot with an approved autopilot in place.
So they can let anyone who under regulations can legally log SIC to sit on the right seat and charge for that "privilege".
Note that in this case the P2F pilot isn't strictly "taking away a job" from anyone else, since an SIC isn't required anyway.
As I said before this is a moral / ethical issue, not really about rules or regulation or even "safety". But morals and ethics are subjective, and what is considered "acceptable practice" changes over time.