It’s a tax-exemption, basically.
More broadly, there are a class of fuels sold in the United States that are dyed a special color. These dyed fuels can be used for off-road purposes, they are sold without the normal excise taxes on fuel. These dyed fuels, one primary use being for home heating, are sold without excise tax because the technical purpose of fuel excise taxes is to fund highway maintenance. The policymakers felt that people buying heating oil to heat their homes in the winter should not have to pay toward road maintenance on the purchase of that fuel.
I’m unclear on whether dyed fuel specifically exists for farming, and I’m also unclear on whether or not the undyed fuel sold without being taxed is included in the OECD fuel subsidy figures, but it may be. But the IRS does give guidance that farmers can buy dyed fuel for farming use, but it must be for “non-taxable” purpose exclusively. So a truck that runs on diesel and is used 50% on the farm and 50% off the farm is not allowed to run on the dyed fuel. If caught using dyed fuel improperly it’s a $1,000 fine. [Or more specifically: Penalty. A penalty is imposed on any person who knowingly uses, sells, or alters dyed diesel fuel or dyed kerosene for any purpose other than a nontaxable use. The penalty is the greater of $1,000 or $10 per gallon of the dyed diesel fuel or dyed kerosene involved. After the first violation, the $1,000 portion of the penalty increases depending on the number of violations. For more information on this penalty, see Publication 510.]
Farmers additionally can buy regular undyed fuel to power farm equipment and vehicles used for farming. The farmer can then get either a tax credit or a refund (the IRS specifies which fuels and uses entitle you to a refund and which entitle you to a tax credit) on the excise taxes they paid on this regular fuel. Basically they get a refund of any taxes paid at the pump when they fill a tractor up with fuel from the local gas station. This appears to amount to around $1bn/year in average benefit to farmers.
The why of it is basically two-fold. One is farmers are heavily favored by Federal legislation, it’s been longstanding Federal policy to make farming as cheap as possible and to give significant tax benefits to farming, agricultural fuel subsidies tie into that. Secondly, like I said before, is the fact that the rationale behind excise taxes on fuels is to maintain the highway system, and the Federal government has generally been amenable to tax exemptions when you’re using fuel specifically for non-road activity.