Remember that every single slave coming into the country came in on NORTHERN-flagged ships. Since the North made profits and then deemed the practice illegal, it would be only fair to return the money they had taken from Southerners.
My ancestors were some of the most active abolitionists there were, and many shed blood/gave their lives in the fight to end slavery. I can't presume to speak for them, but I imagine they would have preferred the slavers themselves to have paid up for that which they had done.
That's an absurd claim. Most slaves were imported before the Revolution, and they came in on foreign-flagged ships.