Could be an oversight.
Or, one could rationalize that medieval understanding of poison is in play here, rather than a more modern one, and certainly a fantasy/medieval understanding of fountain gods is in play - they have a spirit, and vital fluids, and are something that is vulnerable to desecration, so poking it with something poisioned, even a snake bite, might possibly do it harm. Five of nine medieval alchemists agree...
IOW, I could see it either way, really.
PvK