Agreed. I think it's kind of like anti-biotics. If the dose is not heavy and frequent enough to kill all of them, this would serve as a natural selector that filters out those snakes who like mice and are weak at acetaminophen, but let the others survive. The result of this is reducing number of rivals of those survivors, thus giving them better opportunity to live. After several iterations, those who remain there are the ones that you can't use this method to kill.