It's unsurprising that police are screening Craigslist for minors selling their bodies. Prostitutes, after all, admit using Craigslist to sell sex.
What I'm less clear on is whether there are some demographic features the police might be identifying, suggesting why folks in jurisdictions where sex trade is illegal would engage in it. Here, I suppose, my background in policy analysis shows its colors. As with efforts to curb illicit drugs, one sees two main trends: enforcement and education. The enforcement angle isn't interested in why, it's interested in who and where and in obtaining convictions. The education angle may or may not have overlap with enforcement. For example, some needle-exchange programs -- a classic example of harm-reduction theory in practice -- have been threatened with enforcement activity as enabling abuse, despite being intended to mitigate disease risk to the broader population and not merely as a benefit to addicts. On the other hand, where the problematic activity isn't illegal, some activists argue it's easier to find and educate the at-risk population into mainstream behavior -- for example, because fear of enforcement might discourage efforts to access harm-reduction programs. So my interest in data, being driven by an interest in understanding the mechanism(s) and issue(s) involved and thus in developing responses, may be of utter inconsequence to those in the best position to collect the information.
And why would one engage in an illegal sex business? Presumably a person could open business in Nevada or Amsterdam, after all, and enjoy both freedom from prosecution and the benefits of a lawfully regulated trade (e.g., legally-enforceable contracts). Whether it makes the business any more glamorous or satisfying is doubtful, but knowing you can call the police if things go amiss -- and get help rather than a lecture and a set of cuffs -- should be worth something.
I suspect -- though I haven't done a study on it -- that no small fraction of those in the sex industry were drawn for reasons that turn on powerlessness. The link above quotes a 14yo who 'worked' Craigslist since the age of 11 saying "I wanted to feel loved. ... I wanted to feel important." An adult open about her sex-industry modeling career explains that she entered it because illness made her unsuitable for ordinary work. It's not worth overlooking that esteem and powerlessness are issues that affect models and performers outside the sex industry. How many teen girls training for careers in dance are encouraged toward undernourishment and smoking by people (teachers, peers, opinion leaders) who make them hate their appearance or doubt themselves?
Of course, I don't want to be misunderstood that poor self-esteem (or simple financial desperation) explains illegal prostitution. There are folks with other explanations for choosing sex careers (I couldn't find the links I had in mind, hmph). I suspect that, beside the slave trade (apparently alive and well in the US, not just port cities, and not just for sex workers), desperate people perceiving no personal alternatives likely comprise a principal portion of the industry most likely to result in the kinds of unhealthy conduct that makes folks who oppose legalization continue to do so. On the other hand, if the reason to outlaw commercial sex is to protect victims rather than dispense punishment upon a class of unworthies, perhaps some effort to design and implement a harm-reduction-model intervention is worthwhile. (While I understand the argument that it's a victimless crime, this isn't the argument advanced by those supporting prosecutions.)
In the US, this is about as politically plausible for the sex industry as for the drug trade.
 OK, some folks are into cuffs. And the authors of Lady Cop (lyrics) definitely understood this. However, I'm thinking the reason cuffs are considered kink is that it's not mainstream. (Yet!)