I think ice and stones and stuff is out of order. If you look though, the police refer specifically to 'snowballs' which are unlikely to damage property or cars anyway.
Also, i remember such snowball fights when i was a kid and if you were the one getting pelted you would scream 'i dont wanna plaaay' - which the police might take totally literally.
Ahhh, it's no big deal i guess but yeah they have got other things to do other than play 'snow patrol'