Maybe they're not 100% private, but they're not 100% public.
Think of it like a movie at the cinema. Just because someone else paid doesn't mean you get to watch it for free. Perhaps you can find a free torrent/download/stream, but everyone knows it's illegal. The original upload is violating copy right.
An argument could be made if the WG is a danger to others (public interest), but if not, then you are violating the terms of the purchase (hell, you are even with a WG who is a danger, but there's grounds for that being done).
If someone were to post my PG pics, I'd flag it to admin, my public gallery is fine, but my private is very different. But then I have my face clearly shown in my public gallery. Putting up my private would only be to cut into my earnings.
All legally and probably even morally correct, but of little practical importance for the reasons discussed. As you know, if you make something available on the internet (whether free or not) it is available for people to chose what they do with it legally or otherwise and you are generally powerless .. even the 'big boys' in terms of movies etc have not succeeded in stopping what you rightly term illegal.
With regard to this forum as you say you could flag it to admin whom I'm sure would consider under the rules but I suspect we both know what the decision would probably be? In addition his decision would be final and there would be nothing you could do about it.
When Mrs Holliday retired it took us some time to try and remove 'traces' (or for them to remove themselves with time) She had advertised on a number of escort sites which had become largely inactive and impossible to contact the owners. Image searches showed hits in places she didn't even know about. Some escort sites back then (maybe now still) used to steal ads from others to boost their traffic.
Adultwork was obviously easy to take down, but the main issue at the time was with a couple of reports on a national site we are not allowed to mention, where the author had included an email contact address which she had foolishly used on occasion in real life and a google search revealed both. Took many months to get those reports deleted.
Out of curiosity I did a comprehensive search about three years ago and could find nothing left .... but that was a decade on!
As an SP nothing when you advertise is private and everything may become public very easily and beyond your control.