It probably allready is prohibited to throw snow-balls at passing cars (disrupting traffic) and hurtful ones at people (assault, injury, etc)
making specific laws against the touching and flinging of snow is ridiculous and nearly impossible to really define or enforce. What are they gonna do, start writing millions of fines every winter?
What about playing with rocks? Is that prohibited? At least once, as a kid, we had fun throwing pebbles at each others. It hurt a little, but added to the excitement.