1) Why does Indonesia find it difficult to stop the open burning?
Indonesia indeed finds it very difficult to stop open burning. The underlying factor is cost. Simply put, it is much cheaper for these companies to burn to clear land, compared to mechanical methods (about US$5 as compared to around US$250). Of course, slash-and-burn farmers cannot afford such high costs.
However, if it was only a matter of slash-and-burn farmers, the haze would not be such a huge problem. These small-scale farmers are not the primary source of these fires. Research have placed 60-80% of blame on either direct or indirect action of commercial plantations in the past (there has not been any definitive figures for this year yet).
The problem is that most commercial plantations also prefer to use fire, for similar cost-related reasons. These companies however are quite smartly able to avert the risk of being caught and punished for open burning.
For example, some companies hire subcontractors or locals to clear land. When these subcontractors or locals are subsequently caught for burning, these companies can easily avoid blame by claiming (either truthfully or not) that the company did not explicitly instruct them to use fire.
Furthermore, these companies often cultivate healthy relationships with local and central government officials. These relationships come in handy if and when these companies are caught for illegal burning. Due to these connections, these cases will not be publicly exposed, or pursued seriously in court, despite compelling evidence (eg. very low budget reports for land clearing which did not match the expenses required for mechanical clearing, and remnants of oil in jerrycans on site). It is interesting to note that only one oil palm plantation company has ever been successfully prosecuted in court thus far for fires, and even that, with a drastically reduced sentence.
So the combination of the ingenuity of these plantation companies, complemented by the cultivation of good relationships with government officials, make it very difficult for Indonesia to stop open burning, despite laws already existing for that purpose.
2) Is the haze only caused by burning of oil palm or there are burning of other plantations as well?
Most of the haze can be traced back to oil palm plantations. To understand this, we must look back to the type of land involved. Most of the fires in Indonesia occur in peatland areas. Peatland is attractive to oil palm plantations because palms that grow on peat has comparatively higher yield. So most of the plantations opening up on peat are oil palm plantations (despite regulations forbidding the use of peatland for commercial purposes). When drained for use, peat dries quickly and becomes highly flammable. Even if these companies do not deliberately burn for land clearing, drained peat is highly susceptible to accidental fires. Thus, these companies are already placing their concessions at risk of fires by opening up these areas.
We also sometimes hear of fires on other plantations, like pulp and paper or rubber, but these plantations are not usually located on peat, as peatland is not very suitable for these trees. Sometimes plantation owners do use fire to clear land for planting rubber and trees, but the effects are usually not as severe as the burning of peat. When peat burns, it releases carbon-rich, sooty smoke that result in very bad haze, and the fires are very hard to put out. Fires on other types of land do not produce such choking smoke, and are usually easier to put out. Therefore, even though other types of plantations have been found to use fires as well, the effect is not as severe as the burning of oil palm plantations on peatland.