Snowballs can be heavier than softballs or baseballs. We generally don't let people throw those at others to hit them intentionally. In fact, MLB players wear protective helmets just in case of such an incident.
Especially when melting is happening, those wet snowballs can clump together well. But also become very heavy.
Often when the snow is fresh and flaky, it won't even form into a snowball. It turns to powder in your hands.
But who's to say, this snowball is dangerous because it's this weight or that one's OK, because of it's weight? Or that one's too wet or mostly ice.
I'd say if it's not aimed at a person or property, allow it. Ie at a tree in the park. At the ground. At one of the empty playground items. Brick walls. But cars, no. People, no. Normal siding, no. Windows, no.
Like any other rule. Smoking at a gas station is probably OK if you're in the right circumstance. Far enough away from fumes or gas, wind blowing the other way, etc. But are we going to ask people to get all scientific about it or just tell them no smoking near the pumps?