Dirty Secret Alert:
And we are ALL forgetting the one reason that Google doesn't need to bother detecting public PBNs... many of them do something that does Google's job for them. Many (heck probably most that I've seen) public PBNs are built with a finite amount of hosting paid for up front, typically a year. The seller sells his PBN as part of a service with an advertised maximum OBL. Once he fills that OBL why should he pay for another year of hosting for his "lifetime" PBN site? He doesn't... and when hosting runs out the site dies, and your link disappears. And since public PBN links rarely last more than a year, Google doesn't have a problem on their hands. Those who buy into public PBNs too much will show a "link rot" pattern on their linking due to all the constantly dying links and Google can hit their money site algorhythmically if they choose without needing to worry about catching individual PBNs.
ThopHayt