I think this might be part of a larger mechanic around handing over seized contraband to the authorities. At the moment if you come-across something illegal then you either pick it up and become a trafficker or leave it be, but I imagine there might be the possibility to turn it over. The mechanic would need to be able to distinguish between illegal goods you intended to turn in and illegal goods you hoped to trade though. Perhaps when you pick them up, you have to alert the authorities and let them know you're going to turn it in, or at least declare them when you enter the policed system, otherwise you're considered a trafficker. Similarly you then have to then turn it in within a set time period or they'll assume you actually went off and sold them.
If you didn't let them know when you originally found them, and just rock up to a station carrying the contraband, or wait until they're about to scan you after a random spot-check before telling them, then your excuse of 'I was only bringing it back to hand it over' won't fly. The reason you didn't tell them was because you were hoping they wouldn't spot them.
Different authorities are different. Handing slaves over to the Imperials won't do much for your humanitarian reputation, you're effectively selling them. With any contraband, you can only declare it to one authority. If you choose the Feds, but need to fly through Imperial space to turn it over then it's still contraband as far as the Imperials are concerned. If you then decide to hand it over to the Imperials then the Feds will wonder where it went and suspect that you actually sold it. If you find slaves in Imperial space and want to do the good thing then you need to try and sneak out of Imperial space first.
Similarly if you think you can declare the contraband to one authority so that they don't arrest you when they scan you, then sneak off and sell it elsewhere you'll be wanted. You told them you had some contraband and then never turned it in... they'll know what happened.
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