I would think that fault is assessed in the usual way. You can;t be at fault if you are not the cause of the collision. If a car with snow tires runs into a stopped vehicle without them, the vehicle with snow tires should still be at fault and vice versa. Another point to consider, according to Consumer Reports, is that snow tires do not work as well as all season tires on (water) wet or dry pavements. So if you are in a place that salts roads, then only during the time before roads are cleared are the snow tires safer. Also, there are some all season tires that have very good snow traction and most that do not. The November 2009 issue of Consumer Reports has this information. There is no general rule - look at the tests and decide.