And then the pirate, slave or not, plays the "Claim Asylum" card from his deck and the costs are back in your court.
It would also have the nice side effect of dissuading countries to lend their flag to shady companies all over the world.
Realistically, once this becomes a strategem, and a known one, anyone with a boat can enter your waters, hoist the jolly roger, then say "Yeah, take me into asylum." And poof, your immigration procedures evaporate.
Aren't the vast majority of these actually "flags of convenience" and registered in Panama?