Generally, if you receive stolen goods, there are two possibilities: you knew they were stolen (read: a prosecutor can convince a jury you knew), in which case you lose the goods and get prosecuted. Or you didn't know they were stolen (read: prosecutors can't convince a jury you knew), in which case you merely lose the goods.
This may seem unfair from the perspective of the unknowing recipient of stolen goods, but letting them keep the goods is equally unfair from the perspective of the original victim, and would feel like the law giving its blessing to the theft (it would also encourage schemes to knowingly but with plausible deniability receive stolen goods). So there's no solution that is guaranteed to appear fair from all perspectives. But the law generally sides with returning the goods to the original owner, effectively undoing the original theft.
In this case, the "investors" who are being targeted by the suits are also getting to keep their own original capital; they're just being required to hand back the ill-gotten gains. So the remedy is effectively undoing the Ponzi scheme.