Covering oil with sand is a rational step in the cleanup process, it quickly removes the oil from direct impact on the environment, and keeps it from getting washed back out into the gulf if the tides get rough.
If you spilled a bunch of oil in your garage, you might well clean it up by dumping sand or kitty litter on it, and then scooping it up later when the sand/litter had absorbed most of the oil.
Assuming they do indeed scoop it up later.