When explaining it to my friends who were worried about me being mixed up with the "wrong type of people", I always use the analogy (legislation and morales aside...)
You can buy a car from some dude in a parking lot or you can go to a dealership and many other options in between. Like any "business", there is the good, the bad and the ugly.... You just have to exercise caution - do your research and like anything else, buyer beware. That is why moderated sites and review boards are so valuable. Like the MOD has suggested, the lower the level, the more likely the person is being pimped out or supporting a habit.
I have worked at so-called "straight jobs" where I was treated worse than I am now and for not much pay to boot. I also know women who work to support their boyfriends and are treated just as bad or worse than SPs who are pimped out - abuse is not limited to our line of work. But because there is the opportunity to get "fast cash" it is of course going to one of the easiest ways (aside from stealing or some other criminal activity) for girls to support a drug habit or pimp, and of course it's usually the bad things we hear about rather than the good.
Another reason why I say, thank goodness for boards like CERB.