Uncle Sam
<...I have to wonder if part of the reason for such subsidies is to promote farming for export to certain countries, and is thus justified, or at rationalized, as a tool of our foreign policy. Grain sales to Russia, for instance.
I'm sure there are reams of documents with detailed justifications for this spending, same as for every other government spending program, but the simple answer is also the same as for every other government spending program.
It's a way to buy votes to keep them in power.