most of the "devastation" happens to be caused by indian tribes (it is their traditional way of agriculture) and small farmers, who usually don´t have the legal ownership of the land and receive plenty of help from international NGOs in their struggle against large-scale commercial farming.
even though it has been burning for 30 years, the climate in the Amazon has not changed, the amount of rainfall hasn´t declined (there and in other regions which suffer the effect of amazonic air masses) and the the duo cattle-soybeans proved that the amazon can sustain commercial farming in the long run.
Actually the right in Brazil (or what is left of it) truly believes that the Amazon is on the verge of being taken by international NGOs and the UN. The only way to defend the territory is to burn the it.
Soy is apparently becoming very popular is S America as an export crop. They will kill their agricultural industry with that.