I don't really understand why the slavery is such an issue.
Because Bountry Hunters don't care much either way about slavery.
However, there are a LOT of bounties where there's a bigger payoff for returning an acquistion alive than dead (there's one in the EoE book for a start).
So it makes sense there would be a type of BH who specialises in that; Brawl skills, takedowns, non-lethal weapons, etc.
There is one entire house of the Guild that is composed of slavers.