But all fuel in the USA is dirt cheap anyway!
In the UK the price difference between 'red diesel' and normal diesel is huge, red diesel is probably only 20%-25% of the cost - and it's a serious offence using it in a road vehicle. It's called 'red' because it contains a red dye, which will stain large quantities of normal fuel. So if you put it in your cars tank just once, it can be detected even after hundreds of gallons of normal fuel have gone through the tank.
It's not just farms of course, quarries use it as well - anything which is non-road use.