I agree with what most of these guys have said, I really doubt the guy has done it on purpose nor really has any control over it. Fawns will stick tight until the very end and by time they decide to run, it is too late. I've done hay with my uncle for a couple of years and let me tell you, that stuff is thick...nearly impossible to see a nest (which a hen will generally try to hide).

Don't get me wrong, its sad that fawns get killed this way, but there really isn't a whole lot that can be done to prevent it. As for the farmers attitude, somebody else hit it exactly, this is his lifestyle and its going to happen (its probably happened enough for him that is doesn't bother him as much anymore), therefore the 'gruff' reaction you experienced.