The simple truth is most of them that say they do don't, either they don't understand English very well and tick everything or a pimp does the profile and ticks it thinking he will get more customers for the girl. In both cases the girl probably wont allow it once the punter has parted with his money.
Of those that do, I would have to guess that the majority have a violent, controlling pimp who forces them to, as for the remainder I have no idea why.
I think it must be said though, that we shout and scream about how bad BB is and avoid this girl as she does BB, yet a lot of us insist on OWO and CIM, just as dangerous a practice (well maybe not quite as bad, but anything you can get from BB you can get from OWO and CIM is almost as bad as BB for the girl)