Similarly, my players did occasionally bump into shanghaiing operations and even more rarely the far more icky slavery in all it's versions but that was only so the players could shut such activities down.
If a player in my game suggested slaving to their group as a way to make their ship's next mortgage payment, I'd dust off Prison Planet and prepare a nice big hot Reality Sandwichtm for them.
I agree with you, but I wouldn't be that nice. The slaves would get free somehow and take over the ship, killing the PCs. Then the players could play on using the slaves as characters, if I let them.
Or, no matter how good a plan they had, they'd get caught before they could do anything, and hello prison planet. If I was nice. Depending on where the characters were, the version of Imperial Marines could do all sorts of things to them. In one polity IMTU, they'd be fed publicly to shoggoths. Heck, I might do both. Let them run prison planet, and when they get aboard the ship, one at a time they go missing...
I run games to give my friends a chance to play as heroes, not as slavers.
And yes, I've had characters executed before in an SF game. Well, it was supposed to be one character. But two of the other PCs, despite knowing that he was a murderer, helped him escape and they all got on a train...
The characters grabbed some extra tickets, and the conductor punched them. The PCs asked when the next stop was, and the conductor replied there's only one stop for this train. They looked at the other passengers and screamed.
Meanwhile, in the morning, the three were missing, and the trackers were puzzled, because the tracks went right to the remnants of an old train track... The characters were never seen by the living again.
I tell people that I expect the characters to be the heroes as part of the info-dump I give them before they play.