I can't find a NY indictment, but the CA one is easy to find. Backpage principals are charged with:
* Systemic _knowing_ commercial promotion of prostitution.
* An elaborate internal management process focused on concealing evidence of their knowledge of prostitution, including internal employee training.
* _Knowing_ commercial promotion of child sex trafficking, in ads that included photos, in which their policy was to strip out keywords indicative of child sex trafficking while still running the ads.
* A long series of money laundering schemes to collect money from sex traffickers without routing those transactions through banks and credit card processing services.
* An elaborate series of financial moves designed to conceal their assets from prosecutors.
This isn't Craigslist. They ran the site like a criminal conspiracy, which is what it was.
I imagine they've plead guilty because they would have lost calamitously at trial, and the sentences they'd have received after a jury conviction would have been horrific. I'd imagine that if you were Carl Ferrer, you'd be thinking, "the prosecutors are going to show the jury a bunch of ads depicting children known to have been trafficked, with evidence that we knowingly massaged the ad to ensure it could be published for our profit".