Not parent's point. The point was not to conflate Prisoners of War (or Prisoners of Spy War here?) with slaves. Slaves are enslaved to provide labor without rights and payments, are treated as commodity, and are traded for other commodities, with explicit monetary value. In the case of spy prisoners, people who are in jail because they broke some law are traded for other people in jail in another country who broke another law, because they are valuable to their country of origin, and they too want to return to their home country instead of being in jail in a foreign country. They are not used for labor and they do not have monetary value that can be used in a market.
