I only bring my snakes into my own yard to take some pictures. I'm mainly worried about parasites and birds of prey, and dog pee in grass LOL. Stressing the snake out is another, but if the temperature is within their appropriate range, it's fine in the yard.
My biggest reason (even besides what jodan and deborah said) not to bring them into public is that public trips are usually much longer, and that is a stressful experience for a ball python. Stress worsens their immune system. Public trips would only be for our personal pleasure, and I think that is selfish.
Dogs NEED to go out and enjoy the world and exercise. Snakes do not.