I agree with you, that this is a very good idea and it would improve the game experience significantly.
But when I thought twice about this topic, the same abuse nightmares came to my mind. Buying some human commodity near a profitable slave-buying station and convert the passengers...
Hmm three things speak against this nightmare:
Passengers/VIP/scientists etc. are normally sold 2000 credits and above. Selling slaves will bring you 2400 or so on Malta... so the profit wouldn't be that high
Human commodities are sold far away from the two or three profitable stations buying slaves. Namely New London, Cortez, Munich, New Berlin. The profit/time ratio is low
The ID conflict. Slavers aren't allowed to dock on lawful bases.