In a weird twist of events, it was the nobles' lust for beautiful girls that somehow made their lives better.
The pimps learned it the hard way that if they did not have the proper 'papers' for their girls, any powerful lord could snatch their exquisite wares using the excuse of implementing the law.
'I am buying her to give her freedom,' He could claim and the slavers would have to just nod and agree.
So frustrated by this 'injustice' the various slavers got clever and began to make these entertaining girls sign a slave contract when they came of age.
The core point perhaps remained the same- the brothels still owned them, and perhaps it was a bit more expensive initially than simple kidnapping, but these small inconvieneces were more than offset by the fact that it was much easier to do business now.