I have a hunch that police didn't uncover lots of child sex trafficking rings on Backpage, nor were they interested in doing so.
It was far more useful as a honeypot & revenue generator for them through John stings. Setting up a fake Backpage ad for a weekend and raking in $20k through the relatively safe act of fooling non-violent offenders is the best deal out there for police.
At the end of it all you get a nice feel-good headline about how the local PD arrested 40 sex offenders.