Will you blame them, anything to make the movie a hit. In a quicksand you float with respect to your weight and the weight of the liquid and due to the mixture of sand silt and water, chances of floating is higher.
This is how they made us think that if someone should pour petrol on a car and the area surrounding the car, and then throw a lit cigarette on the petrol poured on the floor, it will cause the car to catch fire. I recently learnt how untrue that theory is.
Watched so many of that I can't count, thanks for adding that, anyway it's quite fun to watch sometimes.. now you met a movie freak