I've been thinking the same thing, lately.
When Randsomware was a new idea and seemed to be primarily targeting individuals, it made sense that paying your way out should ultimately be legal. Any law against it would be difficult to enforce, so bad actors would still be incentivized to launch attacks, and only law abiding citizens would be hurt.
But now they're attacking banks, and hospitals, and cities. I have trouble imagining large hospital systems would be able to discretely pay hundreds of thousands of dollars illegally without anyone noticing—and if they did, it would be easy to prosecute.
Ergo, outlawing these payments should very significantly reduce the number of ransomware attacks against these large targets. If no one can pay, there's no incentive to attack.