The buckets most likely contain chemicals to starve the fire of oxygen as water does little to quickly quell a metal and tire fire. Plus they often can't turn the valves on hydrants if they get frozen up. Incedently it isn't smart to put water on an oil or gas fire anyway and at some point they probably couldn't tell if fuel was even coming out. It can flow into and below the snow.
In 1980 I watched some Tampa, Florida idiot firemen hook up a fire hose and start wetting down a 5 tank high octane fuel truck that was center punched by a car that ran a red light. The driver got out but the car struck the valves and tore a hole in the tanger and ignited a fired that burned 40 feet high. Everything was fine until they redirected part of the flame and started fuel flowing down the gutter next to the County Court House with the fire hose. Then the fire caught the vapor from the flowing fuel as it ran down the street. Instead of one torch going upward, now they had a torch and a 200 foot long flame running in the gutter underneath about 10 cars. within minutes the cars were exploding and jumping 10 - 15 feet in the air. Fortunately the military firemen at the airbase arrived in short order with chemicals and foam before the local city firemen burned down the entire down town area. All of this was about 200 feet from me as I watched it unfold from my third floor office window. Surprisingly, because the flame was so directed, the paint on the hood of the car next to and below the flame that tore the hole in the tank was not even burnt.