I don't know snakes as well as I know cats, but Hartz has a flea product labeled for them supposedly not a permethrin, but a different form of it that is supposed to be safe. Nope. Nadda. I have still seen cats with seizure-like tremors and have to be treated as a permethrin toxicity.

If snakes can't be exposed to it, it doesn't matter how low the concentration is, it will still be toxic.

If a big company can get away with it, even with thousands of reports of toxic exposures per year, then it isn't surprising that a single pet store must have had luck with it at one point with no problems, so they tell everyone. Do you really potentially want YOUR snake to be the one that is maybe a little extra sensitive and can't hack even the diluted version and dies?