Nowadays, commercial farmers attend classes, usually by their local Ag Agency, especially if they are a part of a cooperative. We learn the dangers of using chemicals to our soil. If a farmer is educated, he won't want to kill his soil for merely one year's return. So, most farmers will be good stewards of their land. And, if one leases his land for a crop, he will still have control over what is sprayed or applied.
Plus, we have to sign for most pesticides and herbicides (especially if using large amounts), and we are responsible for them if audited by the state. So, there is a little oversight, which is good. I might not appreciate my yahoo neighbor dumping herbicides on his soil where the runoff would affect my land or my drinking water. But, for the most part, all of the farmers I know in my area are smart guys, with a sense of responsibility. But, I try to stick to crops sold for industrial use. It's just less headache. Plus corn and soy bean can be cultivated without most chemicals or even much tilling. This helps preserve the soil from erosion and leaching off the good stuff.