Yes, snow is an insulator. However, that does mean that the ground is on fire beneath it. The land will lose heat to the air (not space), dependent on the temperature gradient. Given enough time, the land surface and air will reach an equilibrium. A layer of snow will slow down the rate of heat transfer. The same layer that prevents cooling at night, prevents heating during the day (where the sun is shining). Soil moisture has the same effect. Hence, at identical temperatures, pipes will freeze slower when snow is present, and proportional to its thickness. Eventually, those pipes will freeze also.