If it is field burning (not undergrowth fires along the road side), it is the normal method used in this part of the world for clearing rice stubble. It is cheap and effective and, while not ecologically the best solution, all that poor villagers can afford. Most of the fields round my area are blackened by the fires; this morning I found the stubble in my favourite birding wood (which is actually a rice paddy with lots of trees) had been burnt... luckily it didn't seem to affect the birds much, as they spend most of their time in the canopy.