Some people think it is funny and acceptable to throw things at people minding their own business riding a bicycle. It ain't funny when you are the target and I doubt that it is legal whether it is a coke bottle, a rock, or even a snowball.
As to the case in question, when the first snowball was thrown at an innocent passerby, or vehicle, the nature of the thing changed. When the police officer arrived and let it be known what he was, the nature of the thing changed. The officer might have been a little quick on the draw, but that doesn't give the crowd the right to give him a hard time.
As I stated before, when I was a kid, we used to do things with snow that would curl your toes and we got away with it, but we knew very well that if we got caught our butts would be in a sling for a long time. We were just lucky that no one to my knowledge ever suffered a lasting injury. It was all fun and a lot of giggles, but it was and is illegal to assault anyone who is an unwilling participant.