"So... why promote these weapons as "snake defense" weapons?"
Because they are useless for anything else.
Why kill a snake? Well, if they are in your backyard and you have kids, it might be a good idea to take one of them out in order to teach the kids. An old guy in our neighborhood did that to teach us since we played in the mountains all day unsupervised (those were the days...). Did he need a Taurus Judge? Nah, if I remember right, he used a coke bottle.
On the other hand, if you live near a nesting/breeding area, you could have a problem that firearms can't solve. If not, they are very beneficial, and only superficial people and children shoot animals in the wild that they can't eat or otherwise use. They evolved here for a reason. If the reason were not sufficient, they would be extinct. For instance, we left snakes alone when I was a kid, we lived in the brush next to a mountain, and we almost never saw a rodent --even though it was a prime spot for them. Fast foward to the city, and we never saw a snake, but finding a rodent was no problem.
For those in the know, look at the differential equations in predator-prey systems. That alone can prove the necessity of snakes. Poisonous or not.
So leave the snakes alone, unless you are hungry or need to teach children up close (sometimes a museum or zoo doesn't do it... That Copperhead from my childhood is burned into my memory though...) It is far more important that the child be able to identify the snake that just bit him or her and know prudent first aid, than it is for the parent to sanitize the world for the child.
And forget about the Taurus Judge if you are older than 12. You can get a decent used pistol and maybe even a shotgun to boot for the price. Save enough for the five dollar snake bite kit though, and carry it everytime you go in the brush.
I think I need to go now... I have a new idea. I am going to invent and market a new firearm specifically made to dispatch poisonous spiders...