Most sex workers agree that their safety is improved dramatically by having the business side of it out in the open. And, there have been studies on sex worker safety that indicate strongly that legal prostitution is safer for everyone; it also reduces coercion when it is not underground (women don't need a pimp, if law enforcement is on their side).
Since I started paying attention a few years ago, I've noticed that most commentary from law enforcement and politicians on the subject of prostitution frames it as "human trafficking" or just "trafficking", even when there were no non-consenting parties involved (and nearly all prostitution is between consenting adults). It is misleading, unethical, and actively harmful to the safety of the people they're claiming to help, but, they do it anyway. The implication that someone is being forced into sexual slavery makes them seem like the good guys in the story...when, in reality, they're mostly just harassing and imprisoning poor women who are making an honest living the best way they know how.
Of course no one wants to see children being sexually abused. No one wants to see children or otherwise at-risk people forced into prostitution. But, that's already illegal; they don't have to victimize all sex workers to pursue and prosecute those crimes.
The kicker here is that the police found these kids who had been forced into sexual slavery because of the ads on this site. Had it been completely underground, they wouldn't have found them, wouldn't have been able to set up a sting, and wouldn't have been able to get the girls out of a dangerous and abusive situation. Now, it goes further underground, where the risk is even higher.