Paying a ransom is like defecting in a prisoner's dilemma: it benefits you at the expense of hurting everyone else who will be stuck in the same situation. Making it illegal just forces everyone to cooperate and is something everyone would prefer.
Should be possible to prosecute under existing laws: you're providing support to a criminal enterprise, sounds illegal already.
Paying a ransom is like defecting in a prisoner's dilemma: it benefits you at the expense of hurting everyone else who will be stuck in the same situation. Making it illegal just forces everyone to cooperate and is something everyone would prefer.
Should be possible to prosecute under existing laws: you're providing support to a criminal enterprise, sounds illegal already.