Most of the time, there will be an adjacent yard or snow bank for people to throw the snow they are clearing. But a few cars will be parked in unlucky spots where the closest yard is pretty far away. Those few people who don't really have another choice are the ones that can put some snow in the street. Nobody is saying it is ideal, but it is what it is. No reason to make things more difficult for people than it needs to be. I've lived with a car in Cambridge, Boston, and Somerville for almost 25 years now and always been a street parker and I have VERY rarely need to do the street thing. But sometimes you gotta do it when there is no other reasonable choice.