Thats the common belief, but if you ask precision driving instructors or people who REALLY know how to drive in the snow they'll tell you thats not actually true. The reason people think FWD is better is because its easier to start from a stop for just that reason, the weight bearing down on the front wheels and the extra stability of pulling not pushing the vehicle. BUT thats where any edge FWD has over RWD ends. When the car is moving and you aren't applying gas, all vehicles are the same in the snow whether they're RWD, AWD, or FWD it doesn't matter. They've still got to turn and they've still got to stop. The drive wheels don't help them do that.
The problem with FWD in the snow is that the drive wheels are also the steering wheels. If the drive wheels on a RWD car loose traction you can still steer right? Meaning you can steer out of a skid if one happens. In a FWD car, if the drive wheels have lost traction or are sliding, you CAN'T steer! All that extra weight to the front (FWD vehicles have way front heavy weight distribution) makes it more likely to break into understeer when cornering than a RWD car, and once that happens in the snow, all control of the car is lost. Its just a sled going wherever momentum takes it.
Another problem with FWD is that you can't use the throttle to help steer the car. In a RWD car you can use the throttle to help steer the car around corners, because when the drive wheels are spinning you can still steer. Can't do that in a FWD because you'd loose the ability to steer.
The differences are subtle, but the RWD definately has more control in slippery conditions. I've driven both RWD and FWD skid cars and RWD cars are easier to bring out of a skid once one happens.
So THAT is why snow tires on a RWD are preferable. You get the going power of the snow tires and the control of the RWD.