In the ENT episode 'Bound' Captain Archer learnt that the apparent slave girls of Orion are actually running the operation. They feign getting sold to wealthy business men, then use their pheromones to mind control their 'owners' and take over their business.
Now this discovery would surely have meant the end of this ruse once exposed. After all, what entrepreneur would willingly buy a slave knowing that within weeks she'd have stolen his business and placed it under the control of the Orion criminal groups?
Yet we know Orion slaving is alive and well in Pike's time at least. Maybe longer. Clearly Archer and the Enterprise crew were convinced somehow to keep quiet about it. Indeed, as Pike himself considers going into business as a slave trader, it seems Star Fleet itself knows about the ruse, and keeps silent in order to get the occasional piece of the action.