Probably because it or its offspring will end up biting someone at some point. Nonpoisonous snakes eat rodents also.My son and I just witnessed the killing of an adult Eastern Diamondback. It was in the middle of a dirt road. A driver in front of us stopped, got out, got a shovel from the back of his truck, then cut the snake's head off and threw it in to the brush! Why on Earth did he feel the need to kill a creature that was not posing an immediate threat?
My dad caught a rattlesnake once, my cousin kept it for a while, then dad let it go way off in the woods. It was neat to see one.