The reason people kill snakes is because they can. Usually, there is no logic for it, whatsoever. I could possibly see a problem when they stray into a neighborhood with kids and pets, but that is it.
Snakes do more good than they do harm. The overpopulation of prarie dogs (think bubonic plague, here) in recent years due to rattlesnake roundups is a good case in point.
My CCW is for those snakes that walk upright on two legs, not for the other kind.