I remember as a kid going to a rattlesnake 'farm' and the guy saying you could drink rattlesnake poison because of how it works. Don't quote me on this, but something like it works by being injected into the bloodstream, not through digestion. He said if you had cavities or somewhere the poison could enter the bloodstream, they one would die from drinking it. But just drinking it wouldn't kill you. Now whether that's the truth or not I don't know, but that's what he said. I guess a Google would find the answer.
Edit: Poisons are substances that are toxic (cause harm) if swallowed or inhaled. Venoms are generally not toxic if swallowed, and must be injected under the skin (by snakes, spiders, etc.) into the tissues that are normally protected by skin in order to be toxic. However, we do NOT recommend drinking venom!
So it is true. So that isn't really a 'miracle' that they do that.
2nd edit: I just saw you said arsenic, not snake venom. Never mind.