On the one hand, sure. On the other, if you're kidnapped for ransom by foreign terrorists or guerrillas (or even just hardened criminals without a political agenda) and no money or consideration at all is allowed to change hands, your odds of survival just went from "not ideal, but OK" to "terrible". Telling people they can't do that is basically telling them their loved one's already dead unless the murderous kidnappers respond well to "pretty please" and agree to take on the risk of a process to return the prisoner without even a token reward.